McCarthy was orphaned at the age of four when both his parents died in the great flu epidemic of 1918. He was raised by his father's parents in Minneapolis, Minnesota, and later by an uncle and aunt. He graduated from Campion Jesuit High School in Prairie du Chien, Wisconsin, in 1932. He attended the School of Foreign Service at Georgetown University, Washington, DC, in 1933, intending to enter into the diplomatic field. He also attended the University of Minnesota, Minneapolis, in 1936 and the Actors Studio New York, New York. McCarthy frequently appeared on Broadway and appeared in nearly 100 movies in a career that spanned seven decades. He also had some starring roles, most notably the horror cult classic Invasion of the Body Snatchers.