Asked by Terre Haute politician and lawyer John Palmer Usher, future Secretary of the Interior under President Abraham Lincoln, to testify in a slander case, Cannon became fascinated with the law. Eventually, he asked Usher if he could study law under him and moved to Terre Haute. At age 19 he traveled to Cincinnati, Ohio, to attend a semester of law school at the University of Cincinnati law school. In 1858, he was admitted to the bar and commenced practice in Terre Haute, Indiana, but was disappointed when Usher refused to offer him a place in his office. That year he relocated to Tuscola, Illinois. His choice of a new hometown was somewhat involuntary, taking place while he was travelling from Shelbyville, Illinois, to Chicago to find more clients for his law firm. During the trip, he ran out of money. He boarded a Chicago-bound train in Mattoon, Illinois; after the train had started, he was asked for his ticket. As Cannon did not have a ticket, he was removed from the train in Tuscola. There, he became State's attorney for the twenty-seventh judicial district of Illinois, holding the position from March 1861 to December 1868. He was one of the charter members of Tuscola's Masonic Lodge No. 332, which was founded on October 2, 1860.