United States Marshal

A United States Marshal, or just Marshal, was a law enforcement title held by some Americans. It was particularly associated with the Old West. The First Doctor, Steven and Dodo once met the Marshals of Tombstone, Arizona and Dodge City, Kansas. (DW: The Gunfighters. The Eleventh Doctor himself was once temporarily appointed a Marshal in Mercy, Nevada in 1870.  (DW: A Town Called Mercy)

Behind the scenes
As may be expected of a British television programme, understanding on the intricacies of American law enforcement aren't well represented in Doctor Who. Little was explained in either televised as to what a Marshal actually was, or how the position differed from that of Sheriff. However, both Donald Cotton and Toby Whithouse chose to stress the fact that their lawmen were Marshals, not Sheriffs.