McKeon was born in Westbury, New York to a show business family; she and her brother Philip did numerous commercials. She also appeared on the soap operas The Secret Storm and Another World. The McKeons moved to Los Angeles in 1975, when Philip got a job as Linda Lavin's son on the television series Alice.

In 1979, McKeon was discovered by a casting director for The Facts of Life after a performance Read more...

Since then, McKeon has starred in many made-for-TV movies, most notably A Cry for Help: The Tracey Thurman Story, in which she portrays an abuse victim. She starred in her own series, Can't Hurry Love, in 1995, which lasted only one season, and which had Mariska Hargitay in a comic role, a few years before Law and Order SVU. In 1997, she starred in another failed sitcom with Jean Smart titled Style Read more...

McKeon married key grip Marc Andrus in June 2003, and in March 2004 gave birth to their daughter, Aurora. According to Lisa Whelchel's website, McKeon had a second baby, a boy, at the end of December 2006.

A devout Roman Catholic, McKeon, with her parents, was granted a private audience with Pope John Paul II while she was on The Facts of Life.