Her best-known role is that of Marion Cunningham on the ABC television sitcom Happy Days, on which she starred from 1974 to 1984 and for which she received two Primetime Emmy Award nominations.

Since the 1990s, Ross has been known for voice-over work on animated television series, including King of the Hill and SpongeBob SquarePants, among others, and recurring roles on The Drew Carey Show, Gilmore Girls, and Brothers & Sisters and guest appearances.

[4] At the age of 13, she changed the spelling of her name from "Marian" to "Marion", as she thought it had a "more pleasing appearance" and would look better on a theatrical program and a theater's marquee.

She found steady work in film, appearing in The Glenn Miller Story (1954), Sabrina (1954), Lust for Life (1956), Lizzie (1957), Teacher's Pet (1958), Some Came Running (1958), and Operation Petticoat (1959).

She appeared on The George Burns and Gracie Allen Show, The Millionaire (1956 episode), Steve Canyon, Perry Mason (The Case of the Romantic Rogue), Buckskin, The Barbara Stanwyck Show, Father Knows Best, The Outer Limits (The Special One), Thriller (U.S. TV series) (The Prisoner in the Mirror), The Brothers Brannagan (two episodes as Diane Warren), The Eleventh Hour, Mannix, Route 66, Mr. Novak, Death Valley Days, Hawaii Five-O, The Brady Bunch, The Fugitive, and Night Court.

In the 1961–62 television season, she played Gertrude Berg's daughter on the CBS sitcom Mrs. G. Goes to College as well as starred as a mail order bride on Rawhide.

Ross' best known role is on the sitcom Happy Days, which aired for 11 seasons on ABC, from 1974 to 1984, having to appear in almost every episode of the series, with the exception of 2.

Despite panning the film, New York Times critic Janet Maslin enthused that, "Marion Ross does a warm, sturdy job as the devoted housekeeper who has been kept too long under Aurora's wing.

[12] That year, Ross played Aunt Lucille in the film Superhero Movie,[13] and in 2009 she appeared in a guest spot on The New Adventures of Old Christine.

In 2010, Ross guest-starred on Nurse Jackie and Grey's Anatomy, and appeared in the Cartoon Network television film Scooby-Doo!

In response, she, Erin Moran, Don Most, Anson Williams, and the estate of Tom Bosley filed a $10 million lawsuit against Paramount, claiming they had not been paid for merchandising revenue.