Baker served as a deputy sheriff in Lincoln County, New Mexico. Embroiled in the Lincoln County War, he rode in the posse that killed John Tunstall on February 18, 1878. Tunstall, a wealthy 24-year-old English cattleman, and banker, along with Alexander McSween, had earlier established a rival business that competed with the powerful Murphy & Dolan Mercantile and Banking Company that monopolized the trade of the county. In February 1878, Murphy & Dolan obtained a court order to seize some of Tunstall’s horses as payment for an outstanding debt. When Tunstall refused to surrender the horses, Lincoln County Sheriff, William Brady, formed a posse led by deputy William Morton to seize them. Riding in the posse were also Jesse Evans, Tom Hill, and Frank Baker. After protesting the presence of the posse on his land, Tunstall was shot in the head. This incident started what became known as the Lincoln County War.
The very next day, Billy the Kid, who had worked for Tunstall as a cattle guard, along with a man named Dick Brewer, went to the Justice of the Peace of the town of Lincoln, John Wilson, who swore out affidavits and warrants for the posse that had killed John Tunstall. Wilson ordered the town constable, Atanacio Martinez, to serve the warrants. Not enthused about confronting Murphy & Dolan, Constable Martinez deputized Billy the Kid and a man named Fred Waite to go with him. When they arrived at the Murphy & Dolan Mercantile, they found it guarded by troops from Fort Stanton and Lincoln County Sheriff William Brady. Backed by the soldiers and his own heavily armed party, Brady not only refused to let Martinez make any arrests; but disarmed the three men and marched them to the jail. Martinez was let go, but Billy the Kid and Fred Waite remained in jail until February 23, missing Tunstall’s funeral.
On March 1, Justice of the Peace John Wilson appointed Dick Brewer as town constable and deputized several others, including Billy the Kid. Once again, the group, who called themselves, the Regulators, were sent to bring in Tunstall’s murderers. On March 6, the Regulators arrested Bill Morton and Frank Baker near the Pecos River, some 60 miles from Lincoln. Three days later, on March 10, 1878, as the posse and their prisoners were making their way back, Bill Morton suddenly snatched Regulator William McCloskey’s pistol and shot him dead. He then began to flee, with Frank Baker closely following. Both were speedily overtaken and killed.