McDermott was born as Mark Anthony McDermott on October 26, 1961 in Waterbury, Connecticut, the son of Diane and Richard McDermott. He is of Italian (from his maternal grandfather), Irish, English, and French descent. Diane was fifteen and Richard was seventeen when McDermott was born; by 1967, the couple had divorced, and Diane and her two children were living with her mother, Avis (Rogers) Marino. On February 9, 1967, Diane was shot and killed. Her death was originally ruled an accident, but police later claimed that evidence they had found would be enough to file murder charges against John Sponza, who had been living with Diane at the time. Sponza told authorities that Diane shot herself (despite the fact that she was right-handed and shot in the back of the left side of her head) after picking up a gun he had been cleaning. Sponza, who police say had ties to organized crime, was killed in 1972; his body was found in the trunk of a car in a Waltham, Massachusetts, grocery store parking lot.