We don’t know. According to The Walking Dead Wiki, ‘Nothing is known about Negan’s life before or as the outbreak began, except that he enjoyed billiards and ping-pong and used to be a used car salesman.’
So aside for once having a job as a used-car salesman, what do you think he did for a living? The first thing out of Mrs. L.A.'s mouth when I asked her was ‘He was a cop.’ She then said she didn’t really think so, but I don’t think it’s beyond the realm of possibilities. And as Mrs. L.A. said, they do like to draw parallels on the show.