Josephine Sarah “Sadie” Earp was born in New York to a Prussian Jewish family; her father was a baker. They moved to San Francisco, where Josephine attended dance school as a girl. When her father had difficulty finding work, the family moved in with her older sister and brother-in-law in a working-class tenement. Josephine ran away, possibly as early as age 14, and traveled to Arizona, where she had an "adventure".

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What is known about Josie is that she had a love of the theatre from a very young age which some believe is the adventure that she “ran away” to Arizona for.  There is some evidence that she lived in Prescott and Tip Top, Arizona Territory under the assumed name of Sadie Mansfield, and worked as a prostitute from 1874 to 1876, before becoming ill and returning to San Francisco. The name Sadie Mansfield was also recorded in Tombstone. Researchers have found that the two names share extremely similar characteristics and circumstances. Later in life Josephine described her first years in Arizona as "a bad dream". What is known for certain is that she traveled to Tombstone using the name Josephine Marcus in October 1880. She wrote that she met Cochise County Sheriff Johnny Behan when she was 17 and he was 33. He promised to marry her and she joined him in Tombstone. He reneged but persuaded her to stay. Behan was sympathetic to ranchers and certain outlaw Cowboys, who were at odds with Deputy U.S. Marshal Virgil Earp and his brothers, Wyatt and Morgan. Josephine left Behan in 1881, before the Gunfight at the O.K. Corral, during which Wyatt and his brothers killed three Cochise County Cowboys. She went to San Francisco in March 1882 and was joined that fall by Wyatt, with whom she remained in a common-law marriage for 46 years until his death.

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The facts about Josephine's life in the Arizona Territory and in Tombstone have been obscured by her legal and personal efforts to keep that period private. Josephine's own story offers a conflicting account of when she first reached Arizona. Her confusing recollection of events show how easily Josephine mixed fact and fiction.

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After Wyatt's death, Josephine collaborated with two of her husband's cousins, Mabel Earp Cason and her sister Vinolia Earp Ackerman, to document her life. The cousins recorded events in her later life, but they found Josephine evasive about the timing and nature of events during her time in the Arizona Territory and Tombstone. She would not even talk about the key events of 1881-82, their key years in Tombstone. The most she would say is that she returned to the Arizona Territory in 1881 and joined Johnny Behan in Tombstone. She said that she had believed Behan was planning to marry her, but he kept putting it off, and she grew disillusioned.

Based on the story she told the Earp cousins, when correlated with other sources, Josephine may have left her parent's home in San Francisco for Prescott, Arizona, as early as October 1874, when she was 13 or 14 years old, not 1879 as she told everyone later on. Cason says she and her sister "finally abandoned work on the manuscript because she [Josie] would not clear up the Tombstone sequence where it pertained to her and Wyatt."