On June 15, 1921, Bennett eloped with Chester Hirst Moorehead of Chicago, a student at the University of Virginia who was the son of an oral surgeon. They were married by a justice of the peace in Greenwich, Connecticut. Bennett was 17 at the time. A New York Times article that reported the elopement noted, "The parents of Miss Bennett were opposed to their marriage at this time solely on account of their youth." The marriage was annulled in 1923.

After some time spent in a convent, Bennett went into the family Business. Independent, cultured, ironic and outspoken, Constance, the first Bennett sister to enter motion pictures, appeared in New York-produced silent movies before a meeting with Samuel Goldwyn led to her Hollywood debut in Cytherea (1924). She abandoned a burgeoning career in silents for marriage to Philip Plant in 1925, but resumed her film career after their divorce, with the advent of talking pictures (1929), and with her delicate blonde features and glamorous fashion style, she quickly became a popular film star.

Bennett's next serious relationship was with millionaire socialite Philip Morgan Plant. Her parents planned a cruise to Europe, taking Constance with them, to separate the couple. As the ship was preparing to leave port, however, the Bennetts saw Plant and his parents boarding, too. A contemporary newspaper article reported, "Now the little beauty and the heir to all the Plant millions were assured a week of the cosy intimacy which an ocean liner affords." In November 1925, the two eloped and were married in Greenwich, Connecticut, by the same justice of the peace who officiated at Bennett's wedding to Moorehead. They divorced in a French court in 1929.

In the early 1930s, Bennett was frequently among the top actresses named in audience popularity and box-office polls. For a short time, she was the highest-paid Actress in Hollywood. So successful was Bennett during this time, that RKO, Bennett's home studio at the time, controlled the careers of actresses Ann Harding and Helen Twelvetrees in a similar manner, hoping to duplicate Bennett's success.

In 1931, Bennett made headlines when she married one of Gloria Swanson's former husbands, Henri le Bailly, the Marquis de La Coudraye de La Falaise, a French nobleman and film Director. She and de la Falaise founded Bennett Pictures Corp. and co-produced two films which were the last filmed in Hollywood in the two-strip Technicolor process, Legong: Dance of the Virgins (1935) filmed in Bali, and Kilou the Killer Tiger (1936), filmed in Indochina. They were divorced in Reno, Nevada in 1940.

In 1932, Bennett returned from Europe with a three-year-old child, whom she claimed to have adopted and named Peter Bennett Plant. In 1942, however, during a battle over a large trust fund established to benefit any descendants of her former husband, Bennett announced that her adopted son actually was her natural child by Plant, born after the divorce and kept hidden to ensure that the child's biological father did not get custody. During the court hearings, the Actress told her former mother-in-law and her husband's widow that "if she got to the witness stand she would give a complete account of her life with Plant. The matter was settled out of court."

Bennett next showed her versatility in the likes of Our Betters (1933), writer/director Gregory La Cava's Bed of Roses (1933) with Pert Kelton, After Tonight (1933) (co-starring with Future husband Gilbert Roland), The Affairs of Cellini (1934), After Office Hours (1935) with Clark Gable, the original Topper (1937, in a career standout as Marian Kerby opposite Cary Grant, a role she repeated in the 1939 sequel, Topper Takes a Trip), the ultimate madcap family comedy Merrily We Live (1938) and Two-Faced Woman (1941, supporting Greta Garbo).

By the 1940s, Bennett was working less frequently in film but was in demand in both radio and theatre. She had her own program, Constance Bennett Calls on You, on ABC radio in 1945-1946. Shrewd Investments had made her a wealthy woman, and she founded a cosmetics and clothing company.

Bennett's fourth marriage was to actor Gilbert Roland. They were married in 1941 and had two daughters, Lorinda and Christina; they divorced in 1946. Later that year, Bennett married for the fifth and final time to US Air Force Colonel (later Brigadier General) John Theron Coulter. After her marriage, she concentrated her efforts on providing relief entertainment to US troops still stationed in Europe, winning military honors for her services. Bennett and Coulter remained married until her death in 1965.

She had a major supporting role in Warner Bros' The Unsuspected (1947) opposite Claude Rains, in which she played the program Director who helps prove that Rains is guilty of murder. She made no films from the early 1950s until 1965 when she made a comeback in the film Madame X (released posthumously in 1966) playing Lana Turner's mother-in-law.

Shortly after filming was completed, on July 25, 1965, Bennett collapsed and died from a cerebral hemorrhage at the age of 60. In recognition of her military contributions, and as the wife of Theron John Coulter, who had achieved the rank of brigadier general, she was buried in Arlington National Cemetery. Coulter died in 1995 and was buried with her.