John Rollins was Sheriff of Angel County.
In high school, he used to sneak a few cigarettes behind the school with the person who would later become the Lieutenant Governor. Formerly a prison guard at San Quentin Penitentiary, he was suborned by inmate Dale "The Whale" Biederbeck and became his henchman. Rollins attempted to frame Adrian Monk for the murder of Frank Nunn, the man responsible for Trudy Monk's murder. He did this by breaking into Monk's apartment, switching Monk's gun barrel with another one while he pocketed the real one, planting the note that would lead Monk to Frank Nunn, and then hiding out in the warehouse and made absolutely certain to position himself behind Monk in trajectory behind a shelf to make it seem as though Monk did fire the gun, and while arresting Monk after kicking the gun on the ground, he switched barrels. Rollins also played a large part in organizing Biederbeck's concurrent plot to murder the Governor of California.
Monk ended up uncovering these plots and Leland Stottlemeyer and Randy Disher arrested Rollins. Knowing full well what awaited a former guard and ex-cop in prison, Rollins immediately turned state's evidence on Biederbeck, confessing to the entire plot in the hopes of minimizing his own sentence.
Presumably while under Biederbeck's payroll, Rollins ended up buying a second house on Catalanca and a BMW, as well as having two separate offshore bank accounts: All from a sheriff's salary, which further fuelled Stottlemeyer's suspicions that Rollins was dirty.