Finally figured out how to transfer my .webarchive HTML signature from Lion to the new .mailsignature file in Mountain Lion. This will work, assuming you already have your HTML signature source code: 1. Open Mail a. Go to Preferences > Signatures b. Create a new signature (no need to customize it here) c. Quit Mail 2. Open Finder a. Go > Go To Folder b. Go to the folder: ~/Library/Mail/V2/MailData/Signatures/ 3. Select your .mailsignature file a. Secondary (right) click b. Open With > Other c. Open with TextEdit d. You will see a few lines of metadata followed by a standard signature code: Code: Content-Transfer-Encoding: 7bit Content-Type: text/html; charset=us-ascii Message-Id: <30069E24-B7CD-4BE7-A550-46CD6E3979F3> Mime-Version: 1.0 (Mac OS X Mail 6.0 \(1485\)) <body style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>Jeremy Marlowe</div><div>firstname.lastname@example.org</div><div><br></div></body><br class="Apple-interchange-newline"> 4. In a new TextEdit file, open your HTML signature source code a. Copy your entire code *EXCEPT* for any <html> and <body> tags at the beginning and end of your code b. Again, DO NOT COPY <html> and <body> tags at the beginning and end of your code 5. Go back to the .mailsignature code open in TextEdit a. Replace the standard signature code below the metadata with your copied source code (DO NOT EDIT OR REMOVE THE METADATA) b. Save c. Close 6. Navigate back to the .mailsignature file you just edited in the ~/Library/Mail/V2/MailData/Signatures/ folder a. Secondary (right) click > Get Info (or command+i) b. Check the "Locked" checkbox (under General)** ** This is CRUCIAL. If you don't lock it, Mail will overwrite your file. 7. Restart Mail a. Go to Preferences > Signatures b. You should see your HTML signature (just like Lion's Mail, images will not display here, but they will show up when you compose your email - assuming, of course, that your image source is still valid) 8. Compose an email and apply the signature to test! I hope this helps! If it doesn't work, the likely causes are that you copied and pasted your code incorrectly or did not lock the .mailsignature file after editing. Be very careful with steps 4, 5 and 6 or this won't work.