I’ve spent a few hours trying to figure out how to create a custom html signature for mail on Mountain Lion. After testing and research, I found the below instructions to be the best method. There are many others, but this method is free and easiest.

1. Open Mail
2. Go to Preferences > Signatures
3. Create a new signature (no need to customize it here)
4. Quit Mail
5. Open Finder
6. Select Go > Go To Folder
7. Go to the folder: ~/Library/Mail/V2/MailData/Signatures/
8. Select the latest .mailsignature file (sort by time if needed)
9. Open file in textwrangler or another editor.
10. Within this file you will notice an open and closed body tag with content in between. Remove that content and insert your html code (without the html, header, and body tags).
11. Save the file
12. Go back to the finder and right click the file and select “Get info”
13. Place a checkmark in “Locked” (you must lock the file so that it doesn’t get overwritten)
14. Open mail

Done