What’s the difference between an Old West marshal and a sheriff?
Lake Orion, Michigan
Generally speaking, U.S. marshals and their deputies dealt with federal issues. The town marshals and their deputies acted as peace officers in the towns. The county sheriffs were usually the highest elected official in the county and their jurisdiction was county-wide. Many times, an officer was a deputy U.S. marshal and could also be a sheriff or town marshal. As you can see, holding both commissions gave him a wider range of authority.
For a good book on frontier government, I recommend Mining Camps: A Study in American Frontier Government by Charles Howard Shinn.