Almost as soon as Norma Jeane Dougherty signed a contract with 20th Century Fox on August 24, 1946, the search for her new stage name was underway. Studio executive Ben Lyon suggested she change her name to Marilyn Monroe. ​​At the time, Lyon thought there were too many possible pronunciations of “Dougherty,” the surname of her soon-to-be ex-husband. The 20-year-old model — who was born Norma Jeane Mortenson and later baptized Norma Jeane Baker — suggested Monroe, another surname on the mother’s side of the family, while Lyon came up with Marilyn because she reminded him of Marilyn Miller, the Ziegfeld Follies Broadway musical star.

Source: Time Magazine