Earp was elected constable of Lamar, Missouri, in 1870. A year later, in 1871, he married Miss Urilla Sutherland. He also joined the Wichita, Kansas police force but, he was discharged in April 1876, following an altercation involving him and another officer. Earp joined the police force in Dodge City and in 1878, he was appointed assistant city marshal under Charles Bassett.
In September 1879, he left Dodge City and three months later reached Tombstone, Arizona, where he became a farmer. After Wyatt's brother, Virgil Earp became city marshal of Tombstone, he recruited Wyatt Earp and Morgan Earp as "special deputy policemen."
Phineas Clanton, Billy Clanton, Tom McLaury and Frank McLaury sold livestock to Tombstone. Wyatt Earp was convinced that the Clanton and McLaury brothers had been selling stolen livestock, and had stolen one of his horses. Earp also came into conflict the sheriff John Behan because of a fight involvoing a woman.