Because I couldn't find any documentation on this at all, anywhere, here is how to make mailto: URLs open in Gmail in

  1. Open Open Preferences. On the "General" pane, set "Default email reader" to
  2. In your Gmail instance, open Preferences. Open the "URL Handlers" pane.
  3. Add a new handler by clicking the + button in the bottom left. Set URL scheme to mailto and set URL replacement to:
    Make sure to include the final colon.

Mailto links should start working immediately, so when you click on an email link on a website, it will open a compose window in your instance. This also provides a hint about how to use the URL handler feature in in general, though I haven’t tried anything else.</p>

Update: This changed in 1.7, I think, without any mention in the Changelog. Instead of following step 3, add a new pattern set in the URL Handler preference dialog. Change the pattern to match mailto:* and change the Javascript block to read:

function transform(inURLString) {
return '' + inURLString;

And you're set!