O'Brien's mother, Gladys Flores (1895–1958), was a
flamenco dancer who often performed with her sister Marissa, who was also a dancer. O'Brien is of half-Irish and half-Spanish ancestry. She was raised Catholic.
Film O'Brien made her first film appearance in Metro-Goldwyn-Mayer's
Babes on Broadway (1941) at the age of four, but it was the following year that her first major role brought her widespread attention. As a five-year-old in
Journey for Margaret (1942), O'Brien won wide praise for her quite convincing acting style, unusual for a child of her age. By 1943, she was considered a big enough star to have a cameo appearance in the all-star military show finale of
Thousands Cheer. Also in 1943, at the age of seven, Margaret co-starred in
You, John Jones!, an MGM short film for the
War Activities Committee of the Motion Pictures Industry, with
James Cagney and
Ann Sothern (playing her parents), in which she dramatically recited President Lincoln's "Gettysburg Address". She played Adèle, a young
French girl, and spoke and sang all her dialogue with a French accent in
Jane Eyre (1943). Arguably her most memorable role was in
Meet Me in St. Louis (1944), opposite
Judy Garland. For her performance in this film as the younger sister "Tootie", she was awarded a special juvenile Oscar at the
17th Academy Awards in 1944. Margaret and
June Allyson were known as "The Town Criers" of MGM. "We were always in competition: I wanted to cry better than June, and June wanted to cry better than me. The way my mother got me to cry was if I was having trouble with a scene, she'd say, 'why don't we have the make-up man come over and give you false tears?' Then I'd think to myself, 'they'll say I'm not as good as June,' and I'd start to cry." O'Brien‘s other successes included
The Canterville Ghost (1944),
Our Vines Have Tender Grapes (1945),
Bad Bascomb (1946) with
Wallace Beery, and the first sound version of
The Secret Garden (1949). She played Beth in the 1949 MGM release of
Little Women, but she was unable to make the transition to adult roles. O'Brien later shed her child star image, appearing on a 1958 cover of
Life magazine with the caption "How the Girl's Grown", and was a mystery guest on the TV panel show ''
What's My Line?'' O'Brien's acting appearances as an adult have been sporadic, mostly in small independent films and occasional television roles. She has also given interviews, mostly for the
Turner Classic Movies cable network.
Television O'Brien gave credit to television for helping her reform and modify her public image. In an interview in 1957, when she was 20, she said: "The wonderful thing about TV is that it has given me a chance to get out of the awkward age — something the movies couldn't do for me. No movie producer could really afford to take a chance at handing me an adult role." On November 20, 1950, O'Brien co-starred with
Cecil Parker in "
The Canterville Ghost", on
Robert Montgomery Presents on TV. She appeared as the mystery guest on ''What's My Line?'' in 1957. On December 22, 1957, O'Brien starred in "The Young Years" on
General Electric Theater. She played the role of Betsy Stauffer, a small-town nurse, in "The Incident of the Town in Terror" on television's
Rawhide. She appeared in "The Sacramento Story" on
Wagon Train in 1958, playing Julie Revere, courted by Robert Horton's Flint McCullough. She made a guest appearance on a 1963 episode of
Perry Mason as Virginia Trent in "The Case of the Shoplifter's Shoe". In 1967, she made a guest appearance on the World War II TV drama
Combat!. Also, in a 1968 two-part episode of
Ironside ("Split Second to an Epitaph"), O'Brien played a pharmacist who (quite the opposite of her usual screen persona) was involved in drug theft and was accessory to attempted murder of star
Raymond Burr's
Ironside. Another rare television outing was as a guest star on the popular
Marcus Welby, M.D. in the early 1970s, reuniting O'Brien with her
Journey for Margaret and
The Canterville Ghost co-star
Robert Young. In 1991, O'Brien appeared in
Murder, She Wrote, season 7, episode "Who Killed J.B. Fletcher?", reuniting O'Brien with her
Tenth Avenue Angel co-star
Angela Lansbury. ==Academy Award==