Officer Milligan was a uniform officer in Dick Tracy's police department. He was a slender, older man, with grey/white hair and a lined face. He was often assigned as the guard at the city's holding cells.
Throughout the history of the strip, there have been multiple officers named Milligan depicted (as well as officers named Mulligan and Gilligan). They are not meant to be the same person, and several of them have been shown being killed in the line of duty.
In "The Blind Side", a prose short story about Tracy written by the strip's current technical advisor (Railroad Police Sgt. Jim Doherty) for the PLAINCLOTHES website, the Milligans are shown to be a family of police officers who have served in the city's police force over several generations.