Corby was born Ellen Hansen in Racine, Wisconsin, the daughter of Danish parents. She grew up in Philadelphia, Pennsylvania. An interest in amateur theater while in high school led her to Atlantic City in 1932 where she briefly worked as a chorus girl. She moved to Hollywood that same year and got a job as a script girl at RKO Studios and Hal Roach Studios, where she frequently worked on the Our Gang Comedies, next to her husband, cinematographer Francis Corby. She held that position for the next twelve years and took acting lessons on the side.
Corby began her career as a writer, working on the Paramount Western Twilight on the Trail and 1947's Hoppy's Holiday. She landed her first acting job in 1945, playing a maid in RKO's Cornered.
In 1948 she received an Academy Award nomination and a Golden Globe for Best Supporting Actress playing a lovelorn aunt in I Remember Mama (1948). Over the next four decades, she worked steadily in both film and television, often playing maids, secretaries, waitresses or gossips. She was a favorite in western films (including Shane, 1953) and had a recurring role as "Henrietta Porter" in the western television series Trackdown (1957 – 1959). Other television appearances included Wagon Train, The Rifleman, I Love Lucy, Tightrope, Meet McGraw (as a maid), The Virginian, Channing, Alfred Hitchcock Presents, Get Smart, Gomer Pyle, The Addams Family, The Beverly Hillbillies and The Andy Griffith Show. She had a recurring role in the 1965-1967 TV series Please Don't Eat the Daisies.
Her most famous role came on CBS in 1971 when she was cast as "Grandma Esther Walton" on the made-for-TV film The Homecoming: A Christmas Story, which served as the pilot for The Waltons. Corby would go on to resume the role on The Waltons, which became a weekly series from 1972–1981, and resulted in several sequel films. For her work in The Waltons, she won her three Emmy Awards and three more nominations as Best Supporting Actress. She left the show early in 1977, due to a massive stroke she suffered, which impaired her speech and severely limited her mobility and function. She returned to the series during the final episode of the 1977-78 season, with her character depicted as also recovering from a stroke. She remained a regular on The Waltons through the end of the 1978-79 season, with Esther Walton struggling with her stroke deficits, as Corby was in real life.
Corby resumed her "Grandma Walton" role in each of the three Waltons reunion movies, during the 1990s.
Corby was married to Francis Corby from 1934 until his death in 1944; some time was spent in Luzerne, PA where she raised two daughters who modelled and also began to act. She was an early practitioner and advocate of the Transcendental Meditation Program, as taught by Maharishi Mahesh Yogi, and appeared with the Maharishi on The Merv Griffin Show in the mid '70s.
She suffered a serious stroke in 1977 but recovered and went on to appear in several television films based on The Waltons. Her stroke was written into the show, with Grandma Walton also suffering a stroke and struggling to regain her speech. Her final role was in the Walton's reunion film A Walton Easter (1997).
Great Mausoleum at Glendale's Forest Lawn Memorial Park Cemetery.