The current acting chief of staff is Mick Mulvaney, who succeeded John Kelly on January 2, 2019.
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Mulvaney has had an interesting career arc, from lawyer to famuly real estate business to state legislature (SC) to Congress (backed by Romney and Club for Growth) to Tea Party and budget hawk/Freedom Caucus, to endorser of Rand Paul in 2015 primaries ... to Trump's Director of OMB ... adding "Acting" Chief of Staff in 2019.
He was best known in Congress for trying to reduce deficits, being more than willing to shut down the government rather than accepting the Paul Ryan deals with RATS in return for increased military spending.