Sheriff McMullin was shot and fatally wounded by a drunken man on the streets of Farmington.

On Monday, Septermber 12, 1880 at 5:30 pm, Sheriff McMullin encountered Henry Horn, a man that he had arrested on previous occasions, on the Main Street sidewalk in front of the courthouse in Farmington. The Sheriff cautioned the drunken Horn against using obscene and abusive language. Horn turned and walked two-thirds the way across the street, pulling his pistol, turned to Sheriff McMullin and fired, striking the sheriff below the right armpit. Sheriff McMullin returned fire striking Horn in the little finger and in the chest but the sheriff's bullet was deflected by a suspender buckle and also fired another round that struck Horn in the hip. Horn fired again and struck Sheriff McMullin in the abdomen, below his ribcage. Horn fell, saw Sheriff McMullin on his feet ready to fire and thew his pistol away in surrender. Sheriff McMullin was taken to his home by friends where he was attended by a physician. Horn was taken into custody.

The suspect had been involved in an incident earlier in the day where friends prevented him from shooting another man at the fairgrounds. The suspect also declared his intent to find and shoot Sheriff McMullin. Horn was charged with first degree murder.

Sheriff McMullin died from his wounds at home three days later on Thursday, September 16th at 3:00 pm.

Sheriff McMullin served as sheriff from 1860 to 1864 and was re-elected in 1877. He was survived by his wife, Ruth, and three daughters. Interred: Masonic Cemetery of Farmington, Missouri.