How to transfer Mac Mail signatures between computers

A theoretical solution...

I find my Mac Mail signatures are stored in the folder at ~/Library/Mail/V6/MailData/Signatures. You may be able to copy the contents of the folder at this location from one computer to another.

1. Hit ⇧⌘G (or Go > Go to Folder... in the Finder) and enter ~/Library/Mail/V6/MailData on the computer that has the signatures you wish to copy. Locate the "Signatures" folder, and copy it over to your target computer.

2. Then (on the target computer), navigate to the same ~/Library/Mail/V6/MailData location, replace the Signatures folder (if present) with the version you copied.

3. Open up Mail and check if the signatures are there.

Two warnings:

1. I haven't tested this, so it might not work/it might mess things up. Make sure you have a backup copy of the signatures folder from your target computer before overwriting it so you can revert if anything goes wrong.

2. If there are already custom signatures on the target computer, they will be overwritten.

Building on Michael Noguera's answer, I was able to transfer just one signature. Some file copying and maybe editing are required.

On the Mac where the signature already exists, look in ~/Library/Mail/V5/MailData/Signatures (yours may not be V5 but pick the most recent).

If the destination Mac has no signatures yet, just copy the entire file AllSignatures.plist. If the destination has some and you want to preserve them, you need to go into the file AllSignatures.plist, find the signature you want by looking at the SignatureName field. Copy that entire XML <dict> ... </dict> block to the destination Mac's AllSignatures.plist file (your destination Mac may have a different version path, i.e. V7, but that's ok).

Now you need to copy the signature itself. In that dict block, there is a field named SignatureUniqueID and the field after it is a long hex string. Look for a file in that directory with the same name as the hex string. Copy that to your destination Mac's Signatures folder.

Quit Mail and restart it. But you may not be quite done.

In my version of Mail (version 13.0), when I opened Preferences --> Signatures, I could see my copied signature in the All Signatures list but it was not associated with one of my mail accounts. I dragged the name of the signature from the middle column to the account name in the left column. Then, when I try to send a new mail from that account, it finally showed up in the possible signatures list.