[2] McCarthy graduated in 1932 from Campion High School in Prairie du Chien, Wisconsin,[8] then attended the University of Minnesota, where he appeared in his first play, Shakespeare's Henry IV, Part 1, and discovered a love of acting.

During his service in World War II in the United States Army Air Forces, McCarthy appeared in the AAF play Winged Victory and its 1944 film adaptation.

He had first performed the role in the London theatrical debut and was the only member of that ensemble to be cast in László Benedek's film adaptation.

McCarthy appeared with Alexis Smith in the NBC anthology series The Joseph Cotten Show in the episode "We Who Love Her" (1956).

He guest-starred in a classic episode of CBS's The Twilight Zone entitled "Long Live Walter Jameson" (1960) as the title character.

[10] In 1971, he guest-starred in the "Conqueror's Gold" episode of Bearcats!, which starred Rod Taylor, with whom McCarthy had appeared in the films A Gathering of Eagles, Hotel, and The Hell With Heroes.

In 1978, McCarthy played a cameo role in a remake of Invasion of the Body Snatchers as a man running through the streets shouting a warning in the same manner as his character did in the original 1956 film.

McCarthy was one of four actors (with Dick Miller, Belinda Balaski and Robert Picardo) often cast by director Joe Dante.

In 1988, McCarthy played R.J. Fletcher, the antagonistic television station owner competing with "Weird" Al Yankovic's George Newman, an aimless young man who is tapped to run a failing UHF television station by his uncle, on the suggestion of his aunt, in the film "UHF" directed by Jay Levey.

In 1996, he played Gordon Fitzpatrick in The Pandora Directive, a full-motion video (FMV) adventure game starring Tex Murphy.

They became best friends, acted together in several more projects, and were believed by some prominent individuals, including Tennessee Williams, Truman Capote[12] and George Whitmore,[13] to have been lovers.