Born in Mount Pleasant (now Farmer City), De Witt County, Illinois, Warner moved with his parents to Clinton, Illinois, in 1843. He attended the common and select schools in Clinton and Lombard University, Galesburg, Illinois. He studied law in Clinton. Enlisted as a private in Company E, Twentieth Regiment, Illinois Volunteer Infantry, June 13, 1861. He was promoted to sergeant June 23, 1861, second lieutenant February 4, 1862, captain and commissary of subsistence February 10, 1865. He was brevetted major March 13, 1865, and was mustered out July 13, 1866. He was graduated from the law department of Harvard University in 1868. He was admitted to the bar the same year and commenced the practice of law in Clinton, Illinois.
Warner was elected as a Republican to the Fifty-fourth and to the four succeeding Congresses (March 4, 1895-March 3, 1905). He served as chairman of the Committee on Revision of the Laws (Fifty-fifth through Fifty-eighth Congresses). He served as Commissioner of Pensions from March 4, 1905, to November 25, 1909. He engaged in business in Clinton, Illinois, as a banker and realty owner and agent. He died in Clinton, Illinois, on March 31, 1925. He was interred in Woodlawn Cemetery.
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