The final years of Monroe's life were marked by illness, personal problems, and a reputation for being unreliable and difficult to work with. The circumstances of her death, from an overdose of barbiturates, have been the subject of conjecture. Though officially classified as a "probable suicide," the possibility of an accidental overdose, as well as the possibility of homicide, have not been ruled out. In 1999, Monroe was ranked as the sixth greatest female star of all time by the American Film Institute. In the years and decades following her death, Monroe has often been cited as a pop and cultural icon.

Monroe was born in the Los Angeles County Hospital on June 1, 1926, as Norma Jeane Mortenson (soon after changed to Baker), the third child born to Gladys Pearl Baker, née Monroe (1902–1984).

Monroe's birth certificate names the father as Martin Edward Mortensen with his residence stated as "unknown". The name Mortenson is listed as her surname on the birth certificate, although Gladys immediately had it changed to Baker, the surname of her first husband and which she still used. Martin's surname was misspelled on the birth certificate leading to more confusion on who her actual father was. Gladys Baker had married a Martin E. Mortensen in 1924, but they had separated before Gladys' pregnancy. Several of Monroe's biographers suggest that Gladys Baker used his name to avoid the stigma of illegitimacy. Mortensen died at the age of 85, and Monroe's birth certificate, together with her parents' marriage and divorce documents, were discovered. The documents showed that Mortensen filed for divorce from Gladys on March 5, 1927, and it was finalized on October 15, 1928.

Throughout her life, Marilyn Monroe denied that Mortensen was her father. She said that, when she was a child, she had been shown a photograph of a man that Gladys identified as her father, Charles Stanley Gifford. She remembered that he had a thin mustache and somewhat resembled Clark Gable, and that she had amused herself by pretending that Gable was her father.

Gladys was mentally unstable and financially unable to care for the young Norma Jeane, so she placed her with foster parents Albert and Ida Bolender of Hawthorne, California, where she lived until she was seven.

While living with the Bolenders, an unusual incident occurred. One day, Gladys came to the Bolenders and demanded that Norma Jeane be released back into her care. Ida knew that Gladys was unstable at the time and insisted that this situation would not benefit Norma Jeane. Unwilling to cooperate, Gladys managed to pull Ida into the yard while she ran inside the house, locking the door behind her. After several minutes, Gladys walked out of the front door with one of Albert Bolender's military duffel bags. To Ida's horror, Gladys had stuffed the now screaming Norma Jeane inside the bag, zipped it up, and proceeded to leave the house. Ida charged towards Gladys and the quarrel resulted in the bag splitting open. Norma Jeane fell out and began weeping loudly as Ida grabbed her and pulled her back inside the house, away from Gladys. This was just one of the many bizarre exchanges between young Norma Jeane and her disturbed mother.

In 1933, Gladys bought a house and brought Norma Jeane to live with her. A few months after moving in, however, Gladys suffered a mental breakdown, beginning a series of mental episodes that would plague her for the rest of her life. In My Story, Monroe recalls her mother "screaming and laughing" as she was forcibly removed to the State Hospital in Norwalk. Norma Jeane was declared a ward of the state, and Gladys' best friend, Grace McKee, became her guardian. It was Grace who had told Monroe that someday she would become a movie star. Grace was captivated by Jean Harlow, and would let Norma Jeane wear makeup and take her out to get her hair curled. They would go to the movies together, forming the basis for Norma Jeane's fascination with the cinema and the stars on screen.

Grace McKee married Ervin Silliman (Doc) Goddard in 1935, and nine-year-old Norma Jeane was sent to the Los Angeles Orphans Home (later renamed Hollygrove), and then to a succession of foster homes. During the time at Hollygrove, several families were interested in adopting her; however, reluctance on Gladys' part to sign adoption papers thwarted those attempts. In 1937, Grace took Norma Jeane back to live with her, Goddard, and one of Goddard's daughters from a previous marriage. This arrangement did not last for long, as Doc Goddard attempted on several occasions to sexually assault her. Disturbed by this, Grace sent her to live with her great-aunt, Olive Brunings in Compton, California. This arrangement also did not last long, as 12-year-old Norma Jeane was assaulted (some reports say sexually) by one of Olive's sons. Biographers and psychologists have questioned whether at least some of Norma Jeane's later behavior (i.e. hypersexuality, sleep disturbances, substance abuse, disturbed interpersonal relationships), was a manifestation of the effects of childhood sexual abuse in the context of her already problematic relationships with her psychiatrically ill mother and subsequent caregivers. In early 1938, Grace sent her to live with yet another one of her aunts, Ana Lower, who lived in the Van Nuys section of Los Angeles. The time with Lower provided the young Norma Jeane with one of the few stable periods in her life. Years later, she would reflect fondly about the time that she spent with Lower, whom she affectionately called "Aunt Ana." Unfortunately, by 1942, the elderly Lower developed serious health problems, and thus Norma Jeane went back to live with the Goddards. It was there where she met a neighbor's son, James Dougherty, and began a relationship with him.

Her time with the Goddards would once again prove to be short. At the end of 1942, Grace and Doc decided to relocate to Virginia, where Doc had received a lucrative job offer. The Goddards decided not to take Norma Jeane with them (the reasoning why was never made clear); thus Grace needed to find a home for her before they moved. An offer from a neighborhood family to adopt Norma Jeane was proposed but Gladys still would not allow it. With few options left, Grace approached Dougherty's mother and suggested that Jim marry her so that she would not have to return to an orphanage or foster care. Dougherty was initially reluctant because Norma Jeane was only sixteen years old, but he finally relented and married her in a ceremony, arranged by Ana Lower, after graduating from high school in June 1942. Monroe would state in her autobiography that she did not feel like a wife; she enjoyed playing with the neighborhood children until her husband would call her home. In 1943, with World War II raging, Dougherty enlisted in the Merchant Marine and was shipped out to the Pacific. Frightened that he might not come back alive, Norma Jeane begged him to try and get her pregnant before he left. Dougherty disagreed, feeling that she was too young to have a baby, but he promised that they would revisit the subject when he returned home. After he shipped out, Norma Jeane moved in with Dougherty's mother.

While Dougherty was in the Merchant Marine, Norma Jeane found employment in the Radioplane Munitions Factory. She sprayed airplane parts with fire retardant and inspected parachutes. During this time, Army photographer David Conover snapped a photograph of her for a Yank magazine article. He encouraged her to apply to The Blue Book Modeling Agency. She signed with the agency and began researching the work of Jean Harlow and Lana Turner. She was told that they were looking for models with lighter hair, so Norma Jeane bleached her brunette hair to a golden blonde.