O'Malley was born in Worcester, Massachusetts to actor Pat O'Malley. She made her screen debut at age 13 months in the film My Old Dutch, starring her father and including her older sister Sheila. It was released the following year in 1926.
In 1943, she played the part of a college girl in the Vincent Sherman directed film Old Acquaintance which starred Bette Davis, Miriam Hopkins and Gig Young. The following year she was in the Charles Vidor directed Cover Girl, a film about a night club owner who was in love with a dancing girl who has other ambitions. In it she played the cigarette girl. The main stars of the film were Rita Hayworth and Gene Kelly. In November 1944, she appeared in the November 23 issue of the Sarasota Herald Tribune, pictured with a small caption "The Most Delightful Target for Our Bomb Sights." She was chosen by bombardiers of the US Army Air Forces. In 1947 she played the part of Dolly in the Alexander Hall-directed musical fantasy Down to Earth, which starred Rita Hayworth and Roland Culver. In 1949 she played the part of Nita in Ladies of the Chorus which starred Marilyn Monroe, Adele Jergens and Rand Brooks.
She had a fairly prominent role in John Ford's 1950 film, Wagon Master. She played the part of pretty Mormon girl Prudence Perkins. Her character Prudence creates an issue due to marrying young Mormon man Sandy instead of Jackson. Prudence is also the cause of a fight between two men who are both perusing her. In 1952, she appeared as Mrs. Norton, a pilot's wife in the Tay Garnett Korean war film One Minute to Zero which starred Robert Mitchum and Ann Blyth.
O'Malley went on to appear in several films and television shows, including Cover Girl, Lady on a Train, Two Tickets to Broadway, Maverick, Rawhide and General Hospital. Her last acting credit came in 1998 when she appeared in the short-lived American crime drama Buddy Faro.
