McCarthy, who was never as well known as her actor brother Kevin McCarthy, was a Communist sympathizer in the 1930s, a Trotskyite, and a vocal opponent of Stalinism. Later, she vigorously opposed the Vietnam war and wrote articles supporting the Vietcong. She contributed to The New York Review of Books, Harper's Magazine, The Nation, and The New Republic.
She served as an editor of The Partisan Review, a literary journal she helped revive in 1937 with its founder, Philip Rahv, with whom she had an affair.
She famously feuded with writer Lillian Hellman, who was also a left-wing writer. Some scholars attribute the feud to differences in degrees of liberalism or professional jealousy. Others say Hellman once tried to seduce McCarthy's lover, Philip Rahv.
In any case, on PBS in 1979, McCarthy told interviewer Dick Cavett that "… every word she writes is a lie, including 'and' and 'the.' " Hellman didn't think the comment was that funny. She sued McCarthy for $2.25 million. The lawsuit never came to trial, though. Hellman died and her executors dropped the case. McCarthy had prepared by fact-checking Hellman's work and finding many errors. She was disappointed not to have her day in court. "I didn't want her to die," said McCarthy. "I wanted her to lose in court. I wanted her around for that."
Writer Nora Ephron wrote a musical, Imaginary Friends (2002), in which she creates a fictional conversation between McCarthy and Hellman. It examines how similar they were and tries to imagine why they hated each other.