Slough was born in Cincinnati, Ohio, son of a steamboat builder. Educated at the University of Cincinnati law school, he practiced law in Cincinnati before being elected to the Ohio General Assembly. While serving there, he struck a fellow assemblyman and was expelled for "an act unbecoming of a gentleman." His constituents reelected him to fill his vacant seat. In 1857, he relocated to Leavenworth, Kansas, opened a law office, and entered the fight against the introduction of slavery into the territory, during the era of Bleeding Kansas. He served in the Wyandotte Constitution convention—making Kansas a "free" state—and, again, settled a legislative difference with his fists. He moved to Denver, Colorado, in 1860 and continued to practice law, becoming one of the city's more distinguished lawyers. With the establishment of the new Colorado Territory in early 1861, he helped organize the courts system, establishment of the bar, and review of professional peers.