Hyland was born Diana Gentner in Cleveland Heights, Ohio. (Some sources indicate Diane Gentner). She made her acting debut at age 19 (in 1955) in an episode of Robert Montgomery Presents. Over the next decade she played numerous guest and supporting roles in various television series, including Naked City, Happy Days, The Eleventh Hour, The Fugitive and The Twilight Zone, before being cast in a featured role in The Chase (1966) with Marlon Brando, Jane Fonda and Robert Redford.