How do you find all the accounts you've logged in to on Steam?

Solution 1:

At least on Windows, if you navigate to C:\Program Files (x86)\Steam\config, there will be a file called loginusers.vdf. Open this with a text editor of your choice, and it will contain entries. Each will have account name, personal name, whether you set it to remember the password, a timestamp, and a flag if the account was the most recent used.

I got this from Steam Community, and tested it on my device. Works great, I had forgotten about some of the accounts I'd used.

I did a quick search and it seems that the location would be ~/Library/Application Support/Steam/config/loginusers.vdf for Mac users. Either way, once you find the Steam folder, it should be relatively straightforward to navigate to it.