Tanner was appointed United States marshal of the Southern District of Illinois by Republican President Chester A. Arthur, but was soon removed by incoming President Grover Cleveland. Now prominent in statewide Illinois politics, Tanner was elected State Treasurer in 1886, and in 1891 he was appointed to the State Railroad and Warehouse Commission. In 1892 he was made Assistant Treasurer of the United States Treasury at Chicago, and was elected chairman of the Illinois State Republican Central Committee in 1894, a committee on which he had previously served as a member (1874–84). As chairman, Tanner was at the head of his party in its heyday of state power and national influence, the boss of a well-oiled political machine that he had helped build. As such, he was instrumental in resuscitating the flagging political fortunes of U.S. Senator Shelby Moore Cullom within his own Republican party. Tanner was a tireless worker and strong-willed party boss. Under his leadership during the 1896 political campaign, the entire Illinois Republican Party state ticket was elected, the party secured majorities in both branches of the state legislature, and eighteen of their twenty-two candidates for congressmen were elected. Tanner himself was elected governor, defeating incumbent John Peter Altgeld by a wide margin. Tanner led up to his inauguration in grand style with his marriage to the socially prominent Cora English on December 30, 1896. She was known for her interest in social reform and her outspoken support of her ideals, views that were shared by her husband. John Tanner's administration was noteworthy in several respects. He placed the state on a sound financial footing, and significant legislation included the establishment of the State Board of Pardons, the State Board of Examiners of Architects, the offices of the State Food Commissioners and the State Commissioners of Game, and the Juvenile Court Act. Western Normal School was established at Macomb (originally a school for teachers, now Western Illinois University). Tanner was also known for his antagonistic relationship with Chicago, and was often at loggerheads with its political bosses, both Democratic and Republican. The mutual hostility came to be symbolized by the "Allen bill", a legislation signed by Tanner which gave control of Chicago's intra-city transportation system to corrupt financier Charles Yerkes. Faced with long-term constraints that would virtually cripple many efforts to develop the city, Chicago was galvanized into instituting internal reforms and ensuring the bill's repeal. Tanner was an active and unbiased patriot. When the Spanish–American War was declared, he was the first governor in the country to have the state militia ready to be called up into national service.