Mustillo was born in Buffalo, New York. He attended American Academy of Dramatic Arts, where he was a contemporary of the Canadian actor, Elias Koteas, and Illeana Douglas. According to the American Academy's 1983 graduation program, Mustillo and Douglas graduated from the Academy's two year program in a ceremony held at NYC's Royale Theatre on April 26th, 1983. They were not asked back to school's third year, known as The Production Company year.
