In 1871, Stevenson defeated incumbent Thomas C. McCreery for his seat in the U.S. Senate after criticizing McCreery for allegedly supporting the appointment of Stephen G. Burbridge, who was hated by most Kentuckians, to a federal position. In the Senate, he opposed internal improvements and defended a constructionist view of the constitution, resisting efforts to expand the powers expressly granted in that document. Beginning in late 1873, Stephenson functioned as the first chairman (later called floor leader) of the Senate Democratic caucus. He did not seek reelection in 1877, returning to his law practice and accepting future Kentucky Governor William Goebel as a law partner. He chaired the 1880 Democratic National Convention and was elected president of the American Bar Association in 1884. He died in Covington on August 10, 1886, and was buried in Spring Grove Cemetery at Cincinnati, Ohio.