Former/old colleague Former/old coworker He's a former colleague of mine ✔ He's an old colleague of mine ✔ He's a former coworker of mine ✔ He's an old coworker of mine ✔ All are correct. Though "former" and "colleague" are more formal English. I'd specifically only use "colleague" if I were referring to someone I worked with in more of a professional environment such as an office (as opposed to something like a fast-food restaurant).

Chiming in here. If you want to be casual, you could absolutely say that this person "used to be someone you worked with". So you're right with that one! That's what a lot of people would use in casual conversation :)

You can say: my former colleague, my former co-worker, my former comrade.

coworker

former colleague