Her theatrical debut was in Sleeping Beauty and the Beast at the Chestnut Theater in Philadelphia; in August 1902, two months before her 20th birthday, she appeared as a singer/comedian in a vaudeville act in Atlantic City.

The dark-haired soubrette, described by a theater reviewer as a "statuesque beauty," attracted notice later that decade for her vocal and comedic talents in The Girl Behind the Counter (1908), The Belle of Brittany (1909), and The Summer Widower (1910).

[5] In 1910, she married millionaire sugar heir and industrialist John Moller Jr and retired from stage work, although she had a small uncredited role as an aristocrat in a 1917 film adaptation of A Tale of Two Cities.

After her husband's sudden death during the 1918 influenza pandemic, Dumont reluctantly returned to the Broadway stage, and soon gained a strong reputation in musical comedies.

[6] In their next Broadway show, Animal Crackers, which opened in October 1928, Dumont again was cast as foil and straight woman Mrs. Rittenhouse, another wealthy, high society widow.

In the Marx Brothers films, Dumont invariably portrayed rich widows whom Groucho would alternately insult and romance for their money: Her role as the excitable, hypochondriacal Mrs. Upjohn in A Day at the Races brought her a Best Supporting Actress Award from the Screen Actors Guild; film critic Cecilia Ager suggested that a monument be erected in honor of Dumont's courage and steadfastness in the face of the Marx Brothers' antics.

Dumont was so important to the success of the Marx Brothers films, she was one of the few people Groucho mentioned in his short acceptance speech for an honorary Oscar in 1974.

"[11] Dumont's acting style, especially in her early films, reflected the classic theatrical tradition of projecting to the back row (for example, trilling the "r" for emphasis).

She also made some television appearances, including a guest-starring role with Estelle Winwood on The Donna Reed Show in the episode "Miss Lovelace Comes to Tea" (1959).

Dumont, usually playing her dignified dowager character, appeared with other film comedians and actors, including Wheeler and Woolsey and George "Spanky" McFarland (Kentucky Kernels, 1934); Joe Penner (Here, Prince 1932, and The Life of the Party 1937); Lupe Vélez (High Flyers, 1937); W.C. Fields (Never Give a Sucker an Even Break, 1941, and Tales of Manhattan 1942); Laurel and Hardy (The Dancing Masters, 1943); Red Skelton (Bathing Beauty, 1944); Danny Kaye (Up in Arms, 1944); Jack Benny (The Horn Blows at Midnight, 1945); George "Gabby" Hayes (Sunset in El Dorado, 1945); Abbott and Costello (Little Giant, 1946); and Tom Poston (Zotz!, 1962).

On February 26, 1965, eight days before her death, Dumont made her final acting appearance on the television program The Hollywood Palace, where she was reunited with Groucho, the week's guest host.