He means "became a US marshal after the time this anecdote took place (since then)", I think. I feel like a lot of temporal career phrases like this have mostly disappeared, like "sometime" meaning formerly/for a time, e.g, "sometime judge in Middlesex county."
Caveat: just speculating based on experience reading 19th century texts. Given that John Brown is writing in a style that is somewhat "clipped," he could mean "due to being a US marshal."
That's related to getting on the California ballot (and possibly name recognition from people who vote for the Peace and Freedom candidate?).