How to apply a layer mask to multiple images in photoshop

Sounds like you want to be using the Batch processing tool built into photoshop (Under File, Automate, Batch).

I believe you will need to point the batch processor to the mask you want to apply and explain how you want photoshop to use it. You will then need to select a directory of images to apply this too.

I'd recommend looking up some tutorials on how to use batch processing on YouTube.

You can’t just leave a mask file open in the workspace and record a sequence of actions that tries to copy/paste from it, as you would do in normal workflow. It creates errors almost immediately.

Through trial and error, I figured out that the correct way to do this is to save the mask as a selection... ‘save selection’... before initiating the batch process, and then ‘load selection’ to the image in the action sequence itself, followed by ‘add mask’. So the action looks something like this:

0. [Image opens from batch file directory]
1. ‘Load Selection’
2. ‘Add Mask’
3. ‘Save’
4. ‘Close’
5. [Next iteration]

Deceptively simple with understanding the use of ‘save/load selection’; horrendously complex or impossible otherwise.