Harlow was born Harlean Harlow Carpenter in Kansas City, Missouri. The name is sometimes incorrectly spelled Carpentier, following later studio press releases. Her father, Mont Clair (or Montclair) Carpenter (1877–1974), son of Abraham L. Carpenter and Dianna (née Beal), was a dentist from a working-class background who attended dental school in Kansas City.
Her mother, Jean Poe Carpenter (née Harlow; 1891–1958), was the daughter of a wealthy real estate broker, Skip Harlow, and his wife, Ella Harlow (née Williams). The marriage was arranged by Jean's father for their underage daughter in 1908. Jean was resentful, and became very unhappy in the marriage. The couple lived in Kansas City in a house owned by Jean's father.
Harlean was nicknamed "The Baby", a name that would stick with her for the rest of her life. She did not learn that her name was actually "Harlean" until the age of five, when she began to attend Miss Barstow's Finishing School for Girls in Kansas City. Harlean and "Mother Jean", as she became known when Harlean became a film star, remained very close. Harlean's mother was extremely protective and coddling, reportedly instilling a sense that her daughter owed everything she had to her. "She was always all mine", she said of her daughter.
When Harlean was at school, her mother filed for a divorce that was finalized uncontested on September 29, 1922. She was granted sole custody of Harlean, who loved the father who would survive her by thirty-seven years. However, Harlean would rarely see him again.
Mother Jean moved with Harlean to Hollywood in 1923 with hopes of becoming an actress, but was too old at 34 to begin a film career.