For some reason Apple hasn't implemented this as a feature in Apple Mail, but it is possible, by a rather long-winded process, to add an HTML signature manually:
- Add a new signature in Mail Settings, and give it some placeholder text in the right-hand pane.
- Still in Mail Settings, drag the signature to your email account and select the signature in the Choose Signature list at the bottom.
- Quit Mail.
- In Finder, open the folder
~/Library/Mail. If your user Library isn't visible (it's hidden by default and Apple recommends keeping it hidden), you can copy that folder path and paste it into Finder's Go To Folder dialog box, accessible from the Go menu. - You'll see one or more folders named
V<num>, where<num>is a number (for instance,V10). Open the folder with the highest number after the V. - Open the MailData folder and then the Signatures folder.
- View the folder in Details view and find the newest file whose name ends in
.mailsignature. Right-click it and open it with TextEdit or a text editor of your choice. - Copy all of the HTML code of the signature you want to import, then in the
.mailsignaturefile select everything from and including the text<body>on down. Then paste your signature. - Save the
.mailsignaturefile and close it. - Right-click the
.mailsignaturefile, choose Get Info, and tick the box marked Locked, to prevent Apple Mail overwriting your signature. - Open Apple Mail and check the signature. If it doesn't look as expected repeat the steps above, paying extra attention to steps 3, 8 and 10.
Source (with screenshots): How to add an HTML signature in Apple Mail (2023)