Cascade Delete Does Not Work on Custom Lookup Fields in Salesforce

Enable Cascade Delete in Lightning Experience

1. Click on Setup
2. Object Manager
3. Select the Object where the custom lookup field is created
4. Open the Lookup Field 
5. Select the Edit Button 
6. Put a tick mark on the option: "Delete this record also. This allows users to delete large numbers of records without regard for sharing or visibility constraints."
7. Hit Save

Enable Cascade Delete in Salesforce Classic

For Standard Objects with Custom Lookup Fields

1. Click on Setup.
2. Look for the Object where the lookup field is created
3. Click on Fields
4. Open the Lookup Field
5. Select the Edit Button
6. Put a tick mark on the option: "Delete this record also. This allows users to delete large numbers of records without regard for sharing or visibility constraints."
7. Hit Save

For Custom Objects with Custom Lookup Fields

1. Click on Setup
2. Open Objects
3. Select the Object where the custom lookup field is created
4. Look for "Custom Fields & Relationship"
5. Open the Lookup Field
6. Select the Edit Button
7. Put a tick mark on the option: "Delete this record also. This allows users to delete large numbers of records without regard for sharing or visibility constraints."
8. Hit Save
   
    

Important Note: 

Salesforce Support will need to set the permission "Enable cascade delete on custom lookup relationships" to see the option, "Delete this record also".