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."
The book Dynamics of Social Change: A Reader in Marxist Social Science, from the Writings of Marx, Engels and Lenin (entire book on archive.org), or a book like it, might be a good idea. It's excerpts from Marx, Engels and Lenin organized thematically, so you can find topics that interest you in the table of contents and it might lead you to essays or further reading once you've finished those excerpts. A bit like spectre's suggestion of a Marxist devotional, I suppose. The only downside of the book I linked is that it was published back in 1970 and that you are reading an edited collection of excerpts, so it's good to be aware of where the authors are coming from (in this case, International Publishers is a publisher affiliated with the Communist Party USA). Still, it's free and and, as someone who easily gets distracted, it's harder to get distracted if you've just set yourself the goal of reading a single short-ish excerpt.
It's just some failed grad student organizing in the wake of occupy, if I'm remembering correctly. After that she got into some fights in DSA committees. Based on how her defenders talk about her in order to justify her shitty takes, you'd imagine she was traveling the land sowing wildcat strikes or in the leadership of a rebellious teamsters local.
That's related to getting on the California ballot (and possibly name recognition from people who vote for the Peace and Freedom candidate?).