I started as a "web developer" without any credentials and have transitioned to "software engineer" without any credentials, though now I'm actually not 100% sure how I did that.

Mostly I just obsessed over functional programming for a long time, and then took the first job I could find doing Haskell (at NYU), then doing an Erlang chat server at a startup, and then doing F# at Jet.com. I guess people assume that only engineers and academics want to do functional languages.