Marilyn's biological father remains unclear. After Gladys returned to Los Angeles, she married Martin Edward Mortensen (1897–1981) on October 11, 1924.[8] They separated on May 26, 1925, and they divorced on August 15, 1928. Martin's father, also named Martin Mortensen, was born in Stavanger, Norway on February 8, 1861, and had emigrated to the United States in 1878 where he married Stella Higgins on October 10, 1894 in Vallejo, California. Their son was born in Vallejo, California.[9]

Norma Jeane's biological father was probably Charles Stanley Gifford, Gladys Baker's shift foreman at Consolidated Film Industries where Baker worked as a film cutter, but her daughter's birth certificate lists Baker's second husband, Martin Edward Mortensen, as the father.[10] Although Mortensen separated from Gladys before her daughter's birth, some biographers speculate he may have been the father.[11] In an interview with Lifetime, James Dougherty, Monroe's first husband, said Norma Jeane believed that Gifford was her father. Whoever the father was, he played no further part in Monroe's life.

Mortensen died in 1981. Amongst his papers were found copies of Norma Jean Baker's birth certificate listing him as the father and copies of his marriage and divorce papers to Gladys Pearl Baker. They were married in 1924 and divorced in 1927 with it becoming final in 1928.