William Warner (June 11, 1840 – October 4, 1916) was an American lawyer and politician based in Kansas City, Missouri.

Warner was elected commander in chief of the Grand Army of the Republic in 1888 for a one-year term.

He was also a member of the Wisconsin Commandery of the Military Order of the Loyal Legion of the United States.

[3] In 1905, Warner was elected as a Republican to the United States Senate, serving from March 18, 1905 to March 3, 1911 in the 60th and 61st Congresses, where he was chairman of the Senate Committee on Mississippi River and Its Tributaries, and served on the Inland Waterways Commission.

Warner resumed the practice of law and was appointed as a civilian member of the Board of Ordnance and Fortifications and a member of the Board of Managers of the National Home for Disabled Volunteer Soldiers.