Born in Albert Lea, Minnesota, she grew up there, and in nearby Waconia and Willmar. At the age of 13, she changed the spelling of her name from "Marian" to "Marion" because she thought it would look better on a marquee. After completing her sophomore year in high school, she moved to Minneapolis and worked as an au pair while studying drama at the MacPhail Center for the Arts and attending Southwest High School. A year later, her family moved to San Diego, California.
Ross enrolled in San Diego State College, where she was named the school's most outstanding actress. After graduation in 1950, she performed in summer theater in La Jolla, California. The director was impressed by her talent, and recommended that she try for work in cinema.
Ross made her 1953 film debut in Forever Female, starring Ginger Rogers and William Holden. She found steady work in film, appearing in The Glenn Miller Story (1954), Sabrina (1954), and Operation Petticoat (1959).
Her career on television began in 1953, when she played the Irish maid on the series Life With Father for two years. Her list of credits spans the history of classic TV, from Burns and Allen Show, Love Boat and Night Court. Ross's most famous role was in the long-running series Happy Days from 1974 to 1984, in which she played Marion Cunningham, Ritchie and Joanie's mother.
Ross has acted on Broadway and in film, but she adores doing TV. In recent years, she played recurring roles as Drew Carey's mother, on The Drew Carey Show; as Grandma Forman, on That '70s Show; and as Lorelei's grandmother on The Gilmore Girls. She also frequently appears on Hollywood Squares, and did voiceovers for Grandma SquarePants on SpongeBob SquarePants.
Ross lives in Los Angeles with actor Paul Michael. Her two children also work in entertainment: Jim Meskimens acting credits include How the Grinch Stole Christmas, and Ellen Plummer is a writer/producer on Friends.