diff -u b/core/modules/media/src/Entity/MediaType.php b/core/modules/media/src/Entity/MediaType.php
--- b/core/modules/media/src/Entity/MediaType.php
+++ b/core/modules/media/src/Entity/MediaType.php
@@ -3,10 +3,8 @@
 namespace Drupal\media\Entity;
 
 use Drupal\Core\Config\Entity\ConfigEntityBundleBase;
-use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Entity\EntityWithPluginCollectionInterface;
 use Drupal\Core\Plugin\DefaultSingleLazyPluginCollection;
-use Drupal\field\FieldStorageConfigInterface;
 use Drupal\media\MediaTypeInterface;
 
 /**
@@ -132,15 +130,6 @@
   protected $field_map = [];
 
   /**
-   * Whether to auto-create source field.
-   *
-   * This is used internally when saving a media type.
-   *
-   * @var bool
-   */
-  protected $auto_create_source_field = FALSE;
-
-  /**
    * {@inheritdoc}
    */
   public function getPluginCollections() {
@@ -224,69 +213,6 @@
   /**
    * {@inheritdoc}
    */
-  public function preSave(EntityStorageInterface $storage) {
-    // If the media handler uses a source field, we'll need to store
-    // its name before saving. We'd need to double-save if we did
-    // this in postSave().
-    $handler = $this->getHandler();
-    if ($this->auto_create_source_field) {
-      $field_storage = $handler->getSourceField($this)->getFieldStorageDefinition();
-      // If the field storage is a new (unsaved) config entity, save it.
-      if ($field_storage instanceof FieldStorageConfigInterface && $field_storage->isNew()) {
-        $field_storage->save();
-      }
-    }
-
-    parent::preSave($storage);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function postSave(EntityStorageInterface $storage, $update = TRUE) {
-    parent::postSave($storage, $update);
-
-    // If the media handler is using a source field, we may need to save
-    // it if it's new. The field storage is guaranteed to exist already
-    // because preSave() took care of that.
-    $handler = $this->getHandler();
-    if ($this->auto_create_source_field) {
-      $field = $handler->getSourceField($this);
-
-      // If the field is new, save it and add it to this media type's view
-      // and form displays.
-      if ($field->isNew()) {
-        // Ensure the field is saved correctly before adding it to the displays.
-        $field->save();
-
-        $entity_type = $field->getTargetEntityTypeId();
-        $media_type = $field->getTargetBundle();
-
-        if ($field->isDisplayConfigurable('form')) {
-          // Use the default widget and settings.
-          $component = \Drupal::service('plugin.manager.field.widget')
-            ->prepareConfiguration($field->getType(), []);
-
-          entity_get_form_display($entity_type, $media_type, 'default')
-            ->setComponent($field->getName(), $component)
-            ->save();
-        }
-        if ($field->isDisplayConfigurable('view')) {
-          // Use the default formatter and settings.
-          $component = \Drupal::service('plugin.manager.field.formatter')
-            ->prepareConfiguration($field->getType(), []);
-
-          entity_get_display($entity_type, $media_type, 'default')
-            ->setComponent($field->getName(), $component)
-            ->save();
-        }
-      }
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getFieldMap() {
     return $this->field_map;
   }
diff -u b/core/modules/media/src/MediaStorageProxyBase.php b/core/modules/media/src/MediaStorageProxyBase.php
--- b/core/modules/media/src/MediaStorageProxyBase.php
+++ b/core/modules/media/src/MediaStorageProxyBase.php
@@ -129,7 +129,7 @@
       return TRUE;
     }
 
-    $source_field_name = $this->getSourceField($media->bundle->entity)->getName();
+    $source_field_name = $this->configuration['source_field'];
 
     // Update thumbnail if we don't have a thumbnail yet, for new items,
     // or if the value of the source field changed.
@@ -179,8 +179,8 @@
       $form['source_field']['#access'] = FALSE;
       $field_definition = $this->fieldTypeManager->getDefinition(reset($this->pluginDefinition['allowed_field_types']));
       $form['source_field_message'] = [
-        '#markup' => $this->t('%field_name field will be automatically created on this type to store the essential information about the media item.', [
-          '%field_name' => $field_definition['label'],
+        '#markup' => $this->t('%field_type field will be automatically created on this type to store the essential information about the media item.', [
+          '%field_type' => $field_definition['label'],
         ]),
       ];
     }
@@ -212,6 +212,13 @@
         $this->configuration[$config_key] = $config_value;
       }
     }
+
+    // If no source field is explicitly set, create it now.
+    if (empty($this->configuration['source_field'])) {
+      $field_storage = $this->createSourceFieldStorage();
+      $field_storage->save();
+      $this->configuration['source_field'] = $field_storage->getName();
+    }
   }
 
   /**
@@ -224,17 +231,16 @@
   /**
    * {@inheritdoc}
    */
-  public function getSourceField(MediaTypeInterface $type, $retry = TRUE) {
-    // If we don't know the name of the source field, we definitely need to
-    // create it.
-    if (empty($this->configuration['source_field'])) {
-      return $this->createSourceField($type);
-    }
-    // Even if we do know the name of the source field, there is no guarantee
-    // that it already exists. So check for the field and create it if needed.
+  public function getSourceField(MediaTypeInterface $type) {
+    // Nothing to do if no source field is configured yet.
     $field = $this->configuration['source_field'];
-    $fields = $this->entityFieldManager->getFieldDefinitions('media', $type->id());
-    return isset($fields[$field]) ? $fields[$field] : $this->createSourceField($type);
+    if ($field) {
+      // Even if we do know the name of the source field, there is no guarantee
+      // that it already exists.
+      $fields = $this->entityFieldManager->getFieldDefinitions('media', $type->id());
+      return isset($fields[$field]) ? $fields[$field] : NULL;
+    }
+    return NULL;
   }
 
   /**
@@ -263,34 +269,23 @@
    *   The field storage definition or NULL if it doesn't exists.
    */
   protected function getSourceFieldStorage() {
-    // If we don't know the name of the source field, we can't get its storage.
-    if (empty($this->configuration['source_field'])) {
-      return NULL;
-    }
-    // Even if we do know the name of the source field, we cannot guarantee that
-    // its storage exists. So check for the storage.
+    // Nothing to do if no source field is configured yet.
     $field = $this->configuration['source_field'];
-    $fields = $this->entityFieldManager->getFieldStorageDefinitions('media');
-    return isset($fields[$field]) ? $fields[$field] : NULL;
+    if ($field) {
+      // Even if we do know the name of the source field, there's no guarantee
+      // that it exists.
+      $fields = $this->entityFieldManager->getFieldStorageDefinitions('media');
+      return isset($fields[$field]) ? $fields[$field] : NULL;
+    }
+    return NULL;
   }
 
   /**
-   * Creates the source field definition for a type.
-   *
-   * @param \Drupal\media\MediaTypeInterface $type
-   *   The media type.
-   *
-   * @return \Drupal\field\FieldConfigInterface
-   *   The unsaved field definition. The field storage definition, if new,
-   *   should also be unsaved.
-   */
-  protected function createSourceField(MediaTypeInterface $type) {
-    // Get the storage or create if it doesn't exists yet.
-    $storage = $this->getSourceFieldStorage();
-    if (is_null($storage)) {
-      $storage = $this->createSourceFieldStorage();
-      $this->configuration['source_field'] = $storage->getName();
-    }
+   * {@inheritdoc}
+   */
+  public function createSourceField(MediaTypeInterface $type) {
+    $storage = $this->getSourceFieldStorage() ?: $this->createSourceFieldStorage();
+
     return $this->entityTypeManager
       ->getStorage('field_config')
       ->create([
@@ -309,10 +304,6 @@
    *   returned. Otherwise, a new, unused one is generated.
    */
   protected function getSourceFieldName() {
-    if ($this->configuration['source_field']) {
-      return $this->configuration['source_field'];
-    }
-
     $base_id = 'field_media_' . $this->getPluginId();
     $tries = 0;
     $storage = $this->entityTypeManager->getStorage('field_storage_config');
diff -u b/core/modules/media/src/MediaStorageProxySourceFieldInterface.php b/core/modules/media/src/MediaStorageProxySourceFieldInterface.php
--- b/core/modules/media/src/MediaStorageProxySourceFieldInterface.php
+++ b/core/modules/media/src/MediaStorageProxySourceFieldInterface.php
@@ -17,15 +17,28 @@
    * @param \Drupal\media\MediaTypeInterface $type
    *   A media type.
    *
-   * @return \Drupal\field\FieldConfigInterface
-   *   The source field definition.
+   * @return \Drupal\field\FieldConfigInterface|null
+   *   The source field definition, or NULL if it does not exist yet or has not
+   *   been set in the handler configuration.
    */
   public function getSourceField(MediaTypeInterface $type);
 
   /**
-   * Gets media storage proxy plugin-specific media item validation constraints.
+   * Creates the source field definition for a type.
    *
    * @param \Drupal\media\MediaTypeInterface $type
+   *   The media type.
+   *
+   * @return \Drupal\field\FieldConfigInterface
+   *   The unsaved field definition. The field storage definition, if new,
+   *   should also be unsaved.
+   */
+  public function createSourceField(MediaTypeInterface $type);
+
+  /**
+   * Gets media storage proxy plugin-specific media item validation constraints.
+   *
+   * @param \Drupal\media\MediaTypeInterface $media_type
    *   A media type. (Because e.g. an oEmbed plugin could be restricted to just
    *   YouTube for a particular media type, so constraints need to be able to be
    *   MediaType-specific.)
diff -u b/core/modules/media/src/MediaTypeForm.php b/core/modules/media/src/MediaTypeForm.php
--- b/core/modules/media/src/MediaTypeForm.php
+++ b/core/modules/media/src/MediaTypeForm.php
@@ -306,10 +306,49 @@
    * {@inheritdoc}
    */
   public function save(array $form, FormStateInterface $form_state) {
-    $this->entity->set('auto_create_source_field', TRUE);
     $status = parent::save($form, $form_state);
     $media_type = $this->entity;
 
+    // If the handler is using a source field, ensure it's properly created.
+    $handler = $media_type->getHandler();
+    if (method_exists($handler, 'getSourceField')) {
+      $source_field = $handler->getSourceField($media_type);
+      if (!$source_field) {
+        $source_field = $handler->createSourceField($media_type);
+
+        /** @var \Drupal\field\FieldStorageConfigInterface $storage */
+        $storage = $source_field->getFieldStorageDefinition();
+        if ($storage->isNew()) {
+          $storage->save();
+        }
+        $source_field->save();
+
+        // Add the new field to the default form and view displays for this
+        // media type.
+        $field_name = $source_field->getName();
+        $field_type = $source_field->getType();
+
+        if ($source_field->isDisplayConfigurable('form')) {
+          // Use the default widget and settings.
+          $component = \Drupal::service('plugin.manager.field.widget')
+            ->prepareConfiguration($field_type, []);
+
+          entity_get_form_display('media', $media_type->id(), 'default')
+            ->setComponent($field_name, $component)
+            ->save();
+        }
+        if ($source_field->isDisplayConfigurable('view')) {
+          // Use the default formatter and settings.
+          $component = \Drupal::service('plugin.manager.field.formatter')
+            ->prepareConfiguration($field_type, []);
+
+          entity_get_display('media', $media_type->id(), 'default')
+            ->setComponent($field_name, $component)
+            ->save();
+        }
+      }
+    }
+
     $t_args = ['%name' => $media_type->label()];
     if ($status === SAVED_UPDATED) {
       drupal_set_message($this->t('The media type %name has been updated.', $t_args));
diff -u b/core/modules/media/tests/modules/media_test_handler/src/Plugin/MediaStorageProxy/Test.php b/core/modules/media/tests/modules/media_test_handler/src/Plugin/MediaStorageProxy/Test.php
--- b/core/modules/media/tests/modules/media_test_handler/src/Plugin/MediaStorageProxy/Test.php
+++ b/core/modules/media/tests/modules/media_test_handler/src/Plugin/MediaStorageProxy/Test.php
@@ -5,6 +5,7 @@
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\media\MediaInterface;
 use Drupal\media\MediaStorageProxyBase;
+use Drupal\media\MediaStorageProxyMetadataSourceFieldHandlerInterface;
 use Drupal\media\MediaTypeInterface;
 
 /**
diff -u b/core/modules/media/tests/src/Functional/MediaCacheTagsTest.php b/core/modules/media/tests/src/Functional/MediaCacheTagsTest.php
--- b/core/modules/media/tests/src/Functional/MediaCacheTagsTest.php
+++ b/core/modules/media/tests/src/Functional/MediaCacheTagsTest.php
@@ -50,7 +50,6 @@
       'handler_configuration' => [],
       'field_map' => [],
       'new_revision' => FALSE,
-      'auto_create_source_field' => FALSE,
     ])->save();
 
     // Create a media item.
diff -u b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
--- b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
+++ b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\media\Functional;
 
 use Drupal\media\Entity\MediaType;
+use Drupal\media\MediaTypeInterface;
 
 /**
  * Trait with helpers for Media functional tests.
@@ -35,7 +36,6 @@
       'handler_configuration' => [],
       'field_map' => [],
       'new_revision' => FALSE,
-      'auto_create_source_field' => FALSE,
     ];
 
     $media_type = MediaType::create($values);
@@ -43,6 +43,23 @@
 
     $this->assertEquals(SAVED_NEW, $status, 'Media type was created successfully.');
 
+    // Ensure that the source field exists.
+    $handler = $media_type->getHandler();
+    if (method_exists($handler, 'getSourceField')) {
+      $source_field = $handler->getSourceField($media_type);
+      if (!$source_field) {
+        $source_field = $handler->createSourceField($media_type);
+        $source_field->getFieldStorageDefinition()->save();
+        $source_field->save();
+
+        $media_type
+          ->set('handler_configuration', [
+            'source_field' => $source_field->getName(),
+          ])
+          ->save();
+      }
+    }
+
     return $media_type;
   }
 
diff -u b/core/modules/media/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php
--- b/core/modules/media/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php
@@ -2,7 +2,9 @@
 
 namespace Drupal\Tests\media\FunctionalJavascript;
 
+use Drupal\field\FieldConfigInterface;
 use Drupal\media\Entity\Media;
+use Drupal\media\Entity\MediaType;
 
 /**
  * Ensures that media UI works correctly.
@@ -68,6 +70,19 @@
     $assert_session->pageTextContains($name);
     $assert_session->pageTextContains($description);
 
+    // We need to clear the statically cached field definitions to account for
+    // fields that have been created by API calls in this test, since they exist
+    // in a separate memory space from the web server.
+    $this->container->get('entity_field.manager')->clearCachedFieldDefinitions();
+    // Assert that the field and field storage were created.
+    $media_type = MediaType::load($machine_name);
+    $handler = $media_type->getHandler();
+    /** @var \Drupal\field\FieldConfigInterface $source_field */
+    $source_field = $handler->getSourceField($media_type);
+    $this->assertInstanceOf(FieldConfigInterface::class, $source_field, 'Source field exists.');
+    $this->assertFalse($source_field->isNew(), 'Source field was saved.');
+    $this->assertFalse($source_field->getFieldStorageDefinition()->isNew(), 'Source field definition was saved.');
+
     /** @var \Drupal\media\MediaTypeInterface $media_type_storage */
     $media_type_storage = $this->container->get('entity_type.manager')->getStorage('media_type');
     $this->testMediaType = $media_type_storage->load(strtolower($name));
@@ -165,14 +180,14 @@
     $assert_session->pageTextContains('The media type ' . $new_name . ' has been deleted.');
 
     // Test type delete prevention when there is existing media.
-    $media_type2 = $this->createMediaType(['auto_create_source_field' => TRUE]);
+    $media_type2 = $this->createMediaType();
     $label2 = $media_type2->label();
     $media = Media::create(['name' => 'lorem ipsum', 'bundle' => $media_type2->id()]);
     $media->save();
     $this->drupalGet('admin/structure/media/manage/' . $media_type2->id());
     $page->clickLink('Delete');
     $assert_session->addressEquals('admin/structure/media/manage/' . $media_type2->id() . '/delete');
-    $assert_session->fieldNotExists('edit-submit');
+    $assert_session->buttonNotExists('edit-submit');
     $assert_session->pageTextContains("$label2 is used by 1 media item on your site. You can not remove this media type until you have removed all of the $label2 media items.");
   }
 
diff -u b/core/modules/media/tests/src/Kernel/BasicCreationTest.php b/core/modules/media/tests/src/Kernel/BasicCreationTest.php
--- b/core/modules/media/tests/src/Kernel/BasicCreationTest.php
+++ b/core/modules/media/tests/src/Kernel/BasicCreationTest.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\media\Kernel;
 
-use Drupal\field\Entity\FieldConfig;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\media\Entity\Media;
 use Drupal\media\Entity\MediaType;
@@ -57,9 +56,16 @@
       'label' => $id,
       'handler' => 'test',
       'new_revision' => FALSE,
-      'auto_create_source_field' => TRUE,
     ]);
     $this->testMediaType->save();
+
+    $source_field = $this->testMediaType->getHandler()->createSourceField($this->testMediaType);
+    $source_field->getFieldStorageDefinition()->save();
+    $source_field->save();
+
+    $this->testMediaType->set('handler_configuration', [
+      'source_field' => $source_field->getName(),
+    ])->save();
   }
 
   /**
@@ -115,90 +121,2 @@
 
-  /**
-   * Tests creating and updating media type programmatically.
-   */
-  public function testProgrammaticMediaTypeManipulation() {
-    // Creating a media type programmatically without specifying a source field
-    // should create one automagically.
-    /** @var FieldConfig $field */
-    $field = $this->testMediaType->getHandler()->getSourceField($this->testMediaType);
-    $this->assertInstanceOf(FieldConfig::class, $field);
-    $this->assertEquals('field_media_test', $field->getName());
-    $this->assertFalse($field->isNew());
-
-    // Saving with a non-existent source field should create it if the related
-    // flag is set.
-    $this->testMediaType->getHandler()->setConfiguration([
-      'source_field' => 'field_magick',
-    ]);
-    $this->testMediaType->set('auto_create_source_field', TRUE);
-    $this->testMediaType->save();
-    $field = $this->testMediaType->getHandler()->getSourceField($this->testMediaType);
-    $this->assertInstanceOf(FieldConfig::class, $field);
-    $this->assertEquals('field_magick', $field->getName());
-    $this->assertFalse($field->isNew());
-
-    // Trying to save without a source field should create a new, de-duped one.
-    $this->testMediaType->getHandler()->setConfiguration([]);
-    $this->testMediaType->set('auto_create_source_field', TRUE);
-    $this->testMediaType->save();
-    $field = $this->testMediaType->getHandler()->getSourceField($this->testMediaType);
-    $this->assertInstanceOf(FieldConfig::class, $field);
-    $this->assertEquals('field_media_test_1', $field->getName());
-    $this->assertFalse($field->isNew());
-
-    // Trying to reuse an existing field should, well, reuse the existing field
-    // even if the auto create flag is set.
-    $this->testMediaType->getHandler()->setConfiguration([
-      'source_field' => 'field_magick',
-    ]);
-    $this->testMediaType->set('auto_create_source_field', TRUE);
-    $this->testMediaType->save();
-    $field = $this->testMediaType->getHandler()->getSourceField($this->testMediaType);
-    $this->assertInstanceOf(FieldConfig::class, $field);
-    $this->assertEquals('field_magick', $field->getName());
-    $this->assertFalse($field->isNew());
-    // No new de-duped fields should have been created.
-    $duplicates = FieldConfig::loadMultiple([
-      'media.' . $this->testMediaType->id() . '.field_magick_1',
-      'media.' . $this->testMediaType->id() . '.field_media_generic_2',
-    ]);
-    $this->assertEmpty($duplicates);
-
-    // If auto-create flag is not set fields shouldn't be created.
-    // ... if an imaginary field is set.
-    $this->testMediaType->getHandler()->setConfiguration([
-      'source_field' => 'field_imaginary',
-    ]);
-    $this->testMediaType->set('auto_create_source_field', FALSE);
-    $this->testMediaType->save();
-    $field = $this->container->get('entity_type.manager')
-      ->getStorage('field_config')
-      ->load('media.' . $this->testMediaType->id() . '.field_imaginary');
-    $field_storage = $this->container->get('entity_type.manager')
-      ->getStorage('field_storage_config')
-      ->load('media.field_imaginary');
-    $this->assertNull($field, 'Field was not automatically created.');
-    $this->assertNull($field_storage, 'Field was not automatically created.');
-
-    // ... if no field is set.
-    $this->testMediaType->getHandler()->setConfiguration([]);
-    $this->testMediaType->set('auto_create_source_field', FALSE);
-    $this->testMediaType->save();
-    $config = $this->testMediaType->getHandler()->getConfiguration();
-    $this->assertEquals('', $config['source_field'], 'Source field value was not automatically populated.');
-    // Check what would the field be named like if it was created.
-    $proposed_field = $this->testMediaType->getHandler()->getSourceField($this->testMediaType);
-    $this->assertInstanceOf(FieldConfig::class, $proposed_field);
-    $this->assertEquals('field_media_test_2', $proposed_field->getName());
-    $this->assertTrue($proposed_field->isNew());
-    $field = $this->container->get('entity_type.manager')
-      ->getStorage('field_config')
-      ->load('media.' . $this->testMediaType->id() . '.' . $proposed_field->getName());
-    $field_storage = $this->container->get('entity_type.manager')
-      ->getStorage('field_storage_config')
-      ->load('media.' . $proposed_field->getName());
-    $this->assertNull($field, 'Field was not automatically created.');
-    $this->assertNull($field_storage, 'Field was not automatically created.');
-  }
-
 }