Sheriff of Mercy

The Sheriff of Mercy was a law enforcement office with jurisdiction in and around the town of Mercy, Nevada. It offered a $500 reward for the capture of a criminal in about 1870. Though it was not known who the Sheriff was, Isaac certainly was not the Sheriff, but a United States Marshal. (TV: A Town Called Mercy)

Behind the scenes
In real life, American sheriffs have jurisdiction over counties, not towns or cities. The existence of a Sheriff of Mercy implies that Mercy is both the name of a county and a town in Nevada. It also explains why the Sheriff was not in town at the time of the Gunslinger's siege.