Wyatt had four full brothers--James, Virgil, Morgan, and Warren--as well as an elder half-brother, Newton, from his father's first marriage.

When his older brothers Newton, James, and Virgil joined the Union Army on November 11, 1861, Wyatt was only 13 years old and too young to enlist, but he tried on several occasions to run away and join the army. Each time, his father found him and brought him home.

In the spring of 1868, the Earps moved to Lamar, Missouri, where Wyatt's father Nicholas became the local constable. When Nicholas resigned on November 17, 1869, to become the Justice of the Peace, Wyatt was appointed constable in his place.

Urilla Sutherland was about to deliver Wyatt's first child when she died suddenly from typhoid fever. Urilla's grave is located at the cemetery in Milford, Missouri, a few miles northeast of Lamar.

In 1871, Wyatt was charged with stealing two horses from William Keys, "each of the value of $100." On April 6, Deputy U.S. Marshal J. G. Owens arrested Earp for the horse theft. He was arraigned on April 14 and bail was set at $500. Rather than wait for trial, Wyatt escaped through the roof of his jail and headed for Peoria, Illinois.

Peoria police raided the home of Jane Haspel in February 1872, arresting four women, along with Wyatt and Morgan Earp. The men were charged with "keeping and being found in a house of ill-fame". Both Earps were arrested for the same crime again on May 11, and Wyatt was arrested once more on September 10, 1872, this time aboard a floating brothel that he owned named the Beardstown Gunboat.

The Heavyweight Championship boxing match between Bob Fitzsimmons and Tom Sharkey was illegal under San Francisco city law. The public bet heavily in Fitzsimmons' favor, but referee Wyatt Earp awarded the fight to Sharkey, claiming that Fitzsimmons had hit below the belt. Virtually no one agreed with Earp's ruling and Fitzsimmons' managers went to court to prevent Sharkey from obtaining the purse. The judge ruled that since the match was illegal the court had no standing, allowing Sharkey to claim the prize.

During the summer of 1878, Ed Morrison and another two dozen cowboys rode into Dodge and shot up the town, galloping down Front Street. Afterwards, they vandalized the Long Branch Saloon and harassed the customers. Hearing the commotion, Earp burst through the front door to find numerous guns pointing at him. Doc Holliday was playing cards in the back and put his pistol to Morrison's head, forcing him and his men to disarm. Earp credited Holliday with saving his life that day, and the two were friends ever after.

Mattie continued to work as a prostitute for at least part of the time that she was involved with Wyatt. Knowledge of her place in Wyatt's life was concealed by Josephine Earp, his later common-law wife, who worked ceaselessly to protect her and Wyatt's reputation in their later years.

Their first confrontation with the Cochise County Cowboys came when U.S. Army Captain Joseph H. Hurst asked the Earp brothers to track down six stolen Army mules. They found the mules at the McLaurys' ranch, along with a branding iron which the Cowboys had used to change the "U.S." brand into "D.8."