Kenin was born in New York City. Her parents divorced when she was young and her mother, Actress Maya Kenin, married character actor John Ryan. Kenin began acting as a child and won her first professional part with a supporting role in the 1972 holiday TV special The House Without a Christmas Tree, which starred Jason Robards and Lisa Lucas. At the age of ten, she was offered the role of Regan MacNeil in The Exorcist. Kenin turned the role down and Linda Blair was eventually cast.

In 1977, Kenin appeared in John Guare's Landscape of the Body at The Public Theater and in the off-Broadway production of Elusive Angel. The following year she played David Janssen's daughter in the television miniseries The Word. In December 1980, she portrayed the role of "Libby" in the touring production of Neil Simon's I Ought to Be in Pictures, starring Bill Macy and Patricia Harty. She also guest starred in several episodes of ABC Afterschool Special, and in the 1982 TV film A Piano for Mrs. Cimino opposite Bette Davis.

At the age of 17, Kenin and her mother moved to Los Angeles after her mother divorced John Ryan. Kenin attended Beverly Hills High School while also maintaining her acting career. In 1979, she was cast in the CBS sitcom Co-Ed Fever. The series was canceled after one episode. The following year, she co-starred in the teen comedy Little Darlings, starring Kristy MacNichol and Tatum O'Neal.

In 1982, Kenin guest starred on episodes of The Facts of Life and Gimme a Break!. In the same year she played, in Honkytonk Man, the part of an aspiring young singer alongside Clint Eastwood as he makes his way to Nashville. One of Kenin's final roles was in the John Hughes film Pretty in Pink (1986), released after her death.

On September 10, 1985, Kenin's body was found in her Manhattan apartment. She had died of an accidental drug overdose.