He moved to Lawrence, Kansas in 1856, to support the "Free-State" movement. He was one of the founders of Emporia, Kansas, where he established the "Kansas News" in 1857. He was secretary of the Free-State convention in 1857 and a member of the Leavenworth constitutional convention in 1859. Plumb studied law and was admitted to the bar in 1861. He was elected to the Kansas House of Representatives in 1862 and was a reporter for the Kansas Supreme Court.