Colasanto attended Bryant University (now located in Smithfield, Rhode Island) and was a decorated veteran of World War II, during which he served as a coxswain in the United States Navy. Around 1954 he was supposed to be an accountant for a company in Saudi Arabia, but he instead attended American Academy of Dramatic Arts. He was of Italian descent.
Although Colasanto was best known for his role as Coach Ernie Pantusso on the sitcom Cheers, he also directed episodes of many television series, notably Hawaii Five-O, Starsky & Hutch, Bonanza, Columbo, and CHiPs. Colasanto appeared in feature films, including The Counterfeit Killer (1968), Fat City (1972), and Alfred Hitchcock's Family Plot (1976).
An in-demand actor and Director, he had been diagnosed in the mid-1970s with heart disease, which was exacerbated by his alcoholism. After twenty years of alcoholism, he became an active member of Alcoholics Anonymous from March 31, 1976, and became sober in the same year. He began having trouble securing directing jobs by the end of the 1970s due to his declining health. His last major film role was a mob boss, Tommy Como in Raging Bull (1980).
Suffering from heart disease, his health worsened by the third season of Cheers. Castmates noticed his weight loss, though the actor chose to keep the severity of his illness a secret. Shortly after the Christmas holiday in 1984, he was admitted to a local hospital due to water in his lungs. Co-star Ted Danson later commented that the veteran actor had difficulty remembering his lines during production of the season.
Colasanto's character was written out of the show as also having died. The fourth-season premiere episode, "Birth, Death, Love and Rice" (1985), deals with Coach's death as well as introducing Colasanto's successor, Woody Harrelson, as Woody Boyd. Colasanto hung a picture of Geronimo in his dressing room. After his death, it was placed on the wall in the bar (part of the Cheers production set) in memory of him. Near the end of the final episode of Cheers, bar owner Sam Malone (Ted Danson) walks over to the picture and straightens it.