In September 1944, Bates signed a contract with Universal Pictures after Cecil Coan introduced her to Producer Walter Wanger. Soon after, she was cast as one of the "Seven Salome Girls" in the 1945 drama, Salome Where She Danced starring Yvonne De Carlo. Around this time, she fell in love with Coan, who was married with two sons and two daughters. In March 1945, Coan divorced his wife and secretly married Bates days later. Bates spent the next few years as a stock Actress, landing bit parts in movies and doing cheesecake layouts for magazines like Yank, the Army Weekly and Life. It was one of those photo sessions that caught the eye of executives at Warner Bros. who signed her in 1947. Warner Bros. highlighted her "girl-next-door" image and her acting career took off. She appeared with some of the biggest stars of the day including Bette Davis in June Bride and Danny Kaye in The Inspector General.
In late 1949, Bates auditioned for the small role of Phoebe in Fox's upcoming All About Eve. In competition for the part was Zsa Zsa Gabor and others, but Bates impressed the producers and was given the part. She made a short but important appearance as the devious schemer, Phoebe, at the end of the film. Bates's image is enshrined in the film's last scene, posing in front of a three-way mirror, while holding the award won by her idol Eve Harrington, played by Anne Baxter. This memorable final scene left critics and audiences intrigued by the young Actress, who they thought would star in a sequel to All About Eve. The Hollywood Reporter said of her performance, "Barbara Bates comes on the screen in the last few moments to more or less sum up the whole action and point of the story. It's odd that a bit should count for so much, and in the hands of Miss Bates all the required points are fulfilled."
After her appearance in All About Eve, Bates co-starred in Cheaper by the Dozen, and its sequel Belles on Their Toes, with Jeanne Crain and Myrna Loy. In 1951, she landed a role opposite MacDonald Carey and Claudette Colbert in the comedy Let's Make It Legal. She co-starred with Donna Reed as the love interests of Dean Martin and Jerry Lewis in the 1953 hit comedy The Caddy.
Despite a seemingly successful career, Bates's life, both on and off screen, started unravelling. She became a victim of extreme mood swings, insecurity, ill health, and chronic depression. In 1954, she won the role of Cathy on the NBC sitcom It's a Great Life, co-starring Frances Bavier as her mother, Amy Morgan, and James Dunn as her uncle, Earl Morgan. After 26 episodes, she was written out of the show due to her erratic behavior, depression and instability. Bates tried to salvage her career and travelled to England to find work. She was signed on as a contract player with the Rank Organisation, only to be replaced in two leading roles before filming began. Bates continued to be too emotionally unstable to work and in 1957, her contract with the Rank Organisation was cancelled.
Upon returning to the United States in 1957, Bates and her husband got an apartment in Beverly Hills. Later that year, Bates made her last film, Apache Territory, which was released in September 1958. She then appeared in two television commercials, one for floor wax and another endorsing a now unknown product with Buster Keaton. In 1960, Bates's husband Cecil Coan was diagnosed with cancer. Bates put her career on hold to care for her ailing husband. The strain eventually became too much for her. She attempted suicide by slashing her wrists and was rushed to Cedars-Sinai Hospital where she soon recovered. She made her final onscreen appearance in an episode of The Saint that aired in November 1962.
In January 1967, Bates's husband Cecil Coan died of cancer. Devastated by his death, Bates's depression worsened and she again became suicidal. Later that year, she returned to Denver and fell out of public view. For a time, Bates worked as a secretary, as a dental assistant, and as a hospital aide. In December 1968 she married for the second time: to a childhood friend, Sportscaster william Reed. Despite her new marriage and location, Bates remained increasingly despondent and depressed.
On March 18, 1969, just months after her marriage to Reed, Barbara Bates committed suicide in her mother's garage by carbon monoxide poisoning. She was 43 years old. She is buried at Crown Hill Cemetery in Jefferson County, Colorado.