Born in Montreal, Quebec, she was the daughter of a Royal Canadian Mounted Policeman. One of the Canadian pioneers in early Hollywood, she began her career as a film extra in 1920, and was already a popular star by 1927 when she converted to Judaism and married MGM's chief of production (and second in command) Irving Thalberg, with whom she had two children.

Shearer was nominated for an Academy Award for Best Actress on six occasions, winning for her role in The Divorcee in 1930. This was one of a series of roles in sophisticated yet racy pre-Code dramas, and Norma was at her best. She was nominated the same year for her role in Their Own Desire, in 1931 for her role in A Free Soul, in 1934 for The Barretts of Wimpole Street, in 1936 for Romeo and Juliet, and in 1938 for Marie Antoinette which was reputedly her favorite role. Marion Davies would later recall that Shearer came to a party at San Simeon in her costume, which required removing the door so she could enter, and four chairs so she could sit at the table.

Shearer was a great beauty, although care was taken in photographing her because she had a lazy eye. Her earlier successful roles were generally those of "modern" women, which was Pre-Code short-hand for sexually uninhibited. As she achieved superstardom, she was relatively restrained in both comedy and tragedy, then settled mostly for "Great Woman" roles. Her screen persona remains indistinct, but particularly after Thalberg's death in 1936 she had a series of surpringly effective performances in smaller and more offbeat vehicles.

Fellow actress Joan Crawford saw the then-Mrs. Thalberg as a hated rival, albeit one she could not defeat, because, as Crawford put it: "She's sleeping with the boss." Shearer and Crawford acted only once together, as bitter rivals in The Women. Their real-life disdain for one another didn't hurt their scenes together.

After Thalberg's death, Shearer embarked on romances with actors George Raft and Mickey Rooney, among others. Rooney said many years later that Shearer "was hotter than a half f****d fox in a forest fire".

She retired from acting in 1942 after the public indifference of her last few films and married Martin Arrouge, a ski instructor twenty years her junior. Confounding the skeptics, they were still happily married at the time of her death (from pneumonia and Alzheimer's disease) either 80 (or 82) years old, although in her declining years she reportedly called Martin "Irving".

She has a Star on the Hollywood Walk of Fame at 6636 Hollywood Boulevard, and is entombed in the Great Mausoleum at Forest Lawn Memorial Park in Glendale, California, in a crypt engraved Norma Shearer Arrouge (next to fellow film star Jean Harlow).