Selden Spencer first held elected office in 1895 when he was voted a member of the Missouri House of Representatives. While in the Missouri House he was Chairman of the Committee on Banks and Banking, as well as on the Judiciary, Ways and Means, Militia, and Rules Committees. From 1897 to 1903 he was a judge of the United States circuit court. At the end of his term on the court Spencer returned to his law practice. He also became heavily involved with the American Bar Association, serving on its executive board and as vice-president in 1914.