Hyland was born Diana Gentner in Cleveland Heights, Ohio. . She made her acting debut in 1955, aged 19, in an episode of Robert Montgomery Presents. Over the next decade she played numerous guest and supporting roles in various television series.
In 1976, she appeared in the television movie The Boy in the Plastic Bubble for which she won a posthumous Emmy Award. The following year, she signed on to play the wife of Dick Van Patten's character in the series Eight is Enough. The first four episodes would be her last role.
In 1977, Hyland was diagnosed with breast cancer, and underwent a mastectomy. Despite the mastectomy, her cancer continued to spread, and her health deteriorated. Hyland and John Travolta, with whom she was romantically involved for one year since 1976, remained together until her death, on March 27, 1977 in Los Angeles. Hyland died in Travolta's arms.