Born Daisy Juliette Baker in Brooklyn, New York she adopted the stage name Margaret and/or Marguerite Dumont. She is remembered mostly for being the comic foil to Groucho Marx in seven of the Marx Brothers movies. Groucho called her practically the fifth Marx brother. (There were in fact five Marx brothers, but only a maximum of four ever performed together.)

Dumont played wealthy high-society widows whom Groucho alternately insulted and romanced for their money. They include Mrs Rittenhouse in Animal Crackers, Mrs Claypool in A Night at the Opera, Mrs Gloria Teasdale in Duck Soup, Martha Phelps in The Big Store, Mrs Susan Dewkesbury in At the Circus, and Emily Upjohn in A Day at the Races. Groucho once said a lot of people believed they were married in real life, but they were not. A typical exchange, from Duck Soup:

Groucho: You might think me a sentimental old fluff ... but would you mind giving me a lock of your hair? Dumont (smitten): A lock of my hair? Why, I had no idea that you ... Groucho: You're getting off easy. I was going to ask for the whole wig! Dumont also endured dialogue about her characters' (and thus her own) stoutish build:

Dumont: I've sponsored your appointment, because I feel you are the most able statesman in all Freedonia. Groucho: Well, that covers a lot of ground. Say, you cover a lot of ground yourself! You'd better move. I hear they're going to tear you down and put up an office building where you're standing! and:

Groucho: Why don't we get married, and take a vacation? I'll need a vacation if we're going to get married. Married! I can see you now, bending over a hot stove. But I can't see the stove! Dumont's presumed ladylike innocence (contrasting with Groucho's perpetual leer) was fodder for Groucho's oft-stated comment that the Brothers had to explain jokes like this to her:

Dumont (after Groucho pulls down a windowshade): What are you doing? Groucho: Defending your honor, which is probably more than you ever did! and:

Dumont: Have you got everything? Groucho: I've never had any complaints yet! Over the course of her lifetime she played in 40 movies, not including some minor silent work. Her first feature film was the Marx Brothers movie The Cocoanuts in 1929 in which she played Mrs Potter, the same role she played in the stage version from which the film was adapted. Her last movie was What a Way to Go! in 1964, in which she played Shirley MacLaine's mother.

She also played the same type with W.C. Fields (in Never Give a Sucker an Even Break), Abbott and Costello, Laurel and Hardy, Jack Benny and Danny Kaye. She also played some dramatic parts.

Just days before her death from a heart attack, she made her final acting appearance on the television programme The Hollywood Palace, where she was reunited onstage with Groucho (that week's guest host) one last time. They performed some material from Animal Crackers. The taped show was actually aired several weeks after her passing. Some sources say it was broadcast on April 10, others say April 17.

In her interviews and press profiles she preserved the myth of her on-screen character - the rich, regal woman who never quite understood the joke - and claimed she had returned reluctantly to acting as a result of widowhood. As a young actress, however, she had specialized in straight female-leads in musical comedies, where the cardinal rule was to make space for the featured comedian.

Her acting style, especially in early films, provides a window into the old-fashioned theatrical style, of projecting to the back row, trilling the "r" for emphasis, etc. Dumont also had a classic-sounding operatic singing voice which the screenwriters would employ from time to time.

The character of the "wealthy dowager" from The Simpsons is based on her work.

On her passing on March 6, 1965, Margaret Dumont was cremated, her ashes stored in the vault at the Chapel of the Pines Crematory in Los Angeles. Dumont was 82 years of age.