Identify Mixpanel user actions that occured before login

You want alias.

From their Javascript API reference:

Use alias() when a unique ID is first assigned (registration), and use identify() to identify the user with that unique ID on an ongoing basis (e.g., each time a user logs in after registering). Do not call identify() at the same time as alias().

From your description, it sounds like, rather than viewing the "About" page anonymously and then logging in, Louie is viewing the "About" page anonymously and then signing up.

In that case, call alias when Louie signs up, and call identify when he logs in after that. That should associate the random, anonymous Mixpanel ID with Louie's new registered user ID.

Note: using this method will mean that, because Louie triggered an event anonymously and then logged in, Louie's anonymous id for that event will not be linked to his distinct id from logging in. If he had signed up after triggering the anonymous event, you would call alias, and they would be linked. This is a known limitation of Mixpanel, unfortunately. From their documentation:

This is the first time he's accessed your site from this device, so we assign a brand new distinct_id to him. He clicks around and then logs in. You should not call mixpanel.alias() in this situation - we haven't seen him on this device, but he is not a new user. ... Instead of calling mixpanel.alias() you should just call mixpanel.identify(). This will remap his phone activity to the original ID he used when signing up for your service, which is the most desirable outcome. This does mean that regrettably the events he fired before logging in will not be associated with him.

More about aliasing in Mixpanel here.