Apple Mail · 7 steps, ~3 minutes

1
Open Mail and choose Mail → Settings (Preferences on older macOS versions).
2
Click the Signatures tab.
3
In the left column, select the email account this signature is for.
4
Click the + button below the middle column to add a new signature, and name it.
5
Uncheck "Always match my default message font," below the signature box — do this BEFORE you paste, while the box is still empty.
6
Select all the placeholder text in the signature box on the right and delete it.
7
Paste your signature into the now-empty box.
Make your signature first →

Good to know

"Always match my default message font" is checked by default, and it is the single most common reason a pasted signature turns into plain black text with no images. Uncheck it before you paste. If you already pasted with it on, don't just uncheck it and hope — clear the box and paste again, which is the fix that reliably works.
While you are writing an email, Mail's compose window can show your signature with more space between the lines than it should have. Mail's editor is drawing it, not your recipient's email app: send yourself a test message and look at the received copy, which is the one that counts.
If the signature lands with oversized icons or images that don't resolve, copy it again from Chrome or Firefox rather than Safari. Safari can hand Mail a bundled copy of the page instead of the plain HTML, and Mail makes a mess of it.
Images can look blank inside the Settings preview — that's normal. Compose a real email and they appear.
Still seeing plain text? Toggle "Place signature above quoted text" off and on, then paste again.
If you created the signature under "All Signatures" instead of a specific account, drag it onto the account in the left column, or it won't show up when you're composing.