McMahon died from heart failure at her home in Monterey, Calif. on March 2, her daughter told the magazine.
Together with the late Dick Clair, McMahon was one of the top comedy writing teams of the '70s and '80s. In addition to The Facts of Life, they made appearances on the Ed Sullivan Show and The Tonight Show before graduating to writing for The Bob Newhart Show and The Mary Tyler Moore Show. Carol Burnett eventually brought them on as writers for her variety series, where they worked from 1973-78.
Born Mary Virginia Skinner in Kansas City, Mo., McMahon moved to New York to study acting under Stella Adler. Her on-screen credits include Dennis the Menace, The Twilight Zone and more.
She is survived by her daughter, Kerry Holden-Dixon, brother Bill and sister-in-law Doris. A memorial service will be held in California.