The uniform would be highly dependent on location, too. A New York marshall would either wear a suit, or a S.W.A.T -type uniform. In the southeastern or western states, it could be the S.W.A.T look, or a more 'old west' shirt, trousers, and cowboy hat. In the northern states, it could even look a bit like Canadian Mounted Police.

In the US, the uniform 'fashion' tends away from the military look with epulets, or decoration, and more toward the functional or comfortable. Except in places like Philidelphia, where they look like postmen.

If this is a Federal Marshall, it would probably be a suit, like the FBI.