The Preacher was a Christian cleric in Mercy, Nevada at least during the period following the American Civil War. In 1870, he prayed for the freedom of the town after it was trapped by the cyborg Kahler-Tek who wanted to terminate Kahler-Jex for his crimes as a war criminal. When Kahler-Tek entered the town, the Preacher was inside the church praying for the population's survival. He protected a small girl who had accidentally toppled a few books, attracting the attention of the cyborg; it spared them.
The Preacher owned a horse that he named Joshua, a name meaning "the deliverer" in the Bible; however, according to the Eleventh Doctor, his horse preferred the name Susan and wanted him to respect the horse's life views. (TV: A Town Called Mercy)