While vacationing in London, MGM mogul Louis B. Mayer happened to see Garson in Old Music; entranced by her elegant manner and flaming red hair, Mayer signed the actress to an MGM contract, showcasing her in the Anglo-American film production Goodbye, Mr. Chips (1939) with Robert Donat.

Garson became MGM's resident aristocrat, appearing most often as co-star of fellow contractee Walter Pidgeon. It was with Pidgeon that she appeared in Mrs. Miniver (1942), a profitable wartime morale-booster which won Oscars for Garson, for supporting actress Teresa Wright, and for the picture itself. Legend has it that Garson's acceptance speech at the Academy Awards ceremony rambled on for 45 minutes; in fact, it wasn't any more than five or six minutes, but the speech compelled the Academy to limit the time any actor could spend in accepting the award.

She reprised the role in the 1950 The Miniver Story with Walter Pidgeon but it turned out to be dreadful, an absolute turkey. For many years when Greer dined at a restaurant and ordered turkey the waiter would bring a copy of The Miniver Story (I made that bit up but you get the sense of how awful this movie is if you haven't seen it!).

Though not overly fond of being so insufferably ladylike in her films, Garson stayed at MGM until her contract expired in 1954; it was surprising but at the same time refreshing to see her let her hair down in the 1956 Western Strange Lady in Town. In 1960, Garson received her seventh Oscar nomination for her astonishingly accurate portrayal of Eleanor Roosevelt in Sunrise at Campobello. After that, Garson was given precious few opportunities to shine in films, though she was permitted to exhibit her still-vibrant singing voice in her last picture, 1967's The Happiest Millionaire.

She was married three times, the first two to Edwin A. Snelson (1932 - 1937) (divorced) and Richard Ney (1943 - 1947) (divorced). Richard Ney was 10 years her junior and played her son in Mrs. Miniver. Their divorce was a stormy affair and with Ney making bitchy remarks about Greer's age during the divorce proceedings effectively signed the death warrant to his own film career as Garson was beloved by the public both in the US and UK. He became a best-selling author of works on finance and investing.

Following her final marriage to Texas oil baron Colonel EE. "Buddy" Fogelson, Garson retired to a ranch in Santa Fe, NM, where she involved herself with various charities. Occasionally Garson returned to make guest appearances on television in ventures ranging from Hollywood Squares, to The Crown Matrimonial, a Hallmark Hall of Fame production. She had to give up even these performances in the early '80s due to chronic heart problems. In 1988, Garson underwent quadruple-bypass surgery. She died of heart failure in Dallas on April 6, 1996.