Over the years creating a decent looking signature in Apple Mail has become more and more convoluted. I have had a number of friends and clients request instructions on how to accomplish this in 10.8 (and now 10.9), so I am posting a write-up for exactly how to accomplish it. For this guide I am going to assume that you know how to (or know someone that does) create a signature using HTML. I will provide a template at the bottom of this page if all you want to do is fill in your information (and custom image).
Using any HTML editor (TextWrangler is free and easy) create your signature as an HTML file. If you would like to use an image in your signature, be sure to upload that to a web server (or Dropbox/GDrive/SkyDrive public folder). For the more advanced users, you can use inline CSS for setting fonts and borders, but I would avoid using it to position items as that can have some unexpected results in various email clients.
Once you have your HTML displaying your signature exactly as you would like it to appear, save the file to your desktop (for this example we will assume it is named “sig.html”).
Open Mail.app and create a new signature. It doesn’t matter what is in this signature, we are going to be replacing it with the one you created. We just need mail to create the proper file for itself. Now quit Mail.
You now want to navigate to “~/Library/Mail/V2/MailData/Signatures”. To do this, open Terminal and type:
cd ~/Library/Mail/V2/MailData/Signatures open .
You should see a file with the extension .mailsignature – if you do not, re-open mail and recreate a new dummy signature.
You now want to open the .mailsignature file using your HTML editor. You will notice a lot of existing information in this file. Find the line that looks like:
Mime-Version: 1.0 (Mac OS X Mail…
You do not want to touch this line or anything above it.
Open sig.html in your HTML editor if you closed it and copy the HTML out of this file. Paste the HTML into the .mailsignature file, replacing the content just below the “Mime-Version” line. Now save this file and close it.
Now this is the part that stymied me for a while. In prior versions of the OS, that was all that was needed. However, now if you were to open mail and quit it, the next time you open it your signature would be gone. This is because for some reason unbeknownst to me, Mail overwrites the contents of that file every time it is closed. Obviously we don’t want that to happen, so we need to tell OSX that it doesn’t have permission to do so. We will do so by locking the file. The easiest way to accomplish this is to right click on the file, select “Get Info” and check the “Locked” checkbox in the window that pops up.
Click here for an example signature. To see the HTML you can view the page source, or just save the page directly to your desktop and open it with an HTML editor.