Manual Masking

As Kevin has said, selections can be saved.  If, by manual masking you mean Quick Mask, then turn it off to reveal the marching ants, and go Select > Save Selection.

If you were masking by painting into a Layer Mask, then just save the file as a .PSD.  If you were not using a Layer Mask, then when you see the marching ants, click on the Layer Mask icon to add one.

The way I save selections is to add a layer and fill the selection with a solid color (usually black).  I park the layer out of the way, or just turn it off.  If I need that selection again I Ctrl/Cmd click the thumbnail of the selection layer to ro load it.

Another way to save selections is in the Channels panel.  If you select a layer that has a Layer Mask, that mask will appear as an additional channel. Indeed, when you use Select > Save Selection that selection is reflected by a new channel in the Channels panel. 

When you deselect the layer with the Layer Mask, the new channel will vanish, but if you drag that channel to the New Channel icon at the bottom of the Channels panel, then you'll have essentially saved the selection and Ctrl click on the channel to load it.

How are you masking? You can stop and start manually creating a layer mask or vector mask at any time.

But if you are drawing a selection or a QuickMask, you do have to either finish it or drop it before you close the document.

If you’re using the Select > Select and Mask command, you can partially work and then choose the command again later to finish, if you first repeat the exact same selection that was active when you first used that command.

Hi @Ron289184022o4a you mean painting on a layer mask?

You can always save and then return later to edit to a mask- they are considered non-destructive.