In 1910, Tanner was appointed First Deputy Attorney General of New York, in charge of the New York City office. He was a delegate to the 1912 and 1916 Republican National Conventions. He served as chairman of the New York Republican State Committee from 1914 to 1917. He was a delegate to the 1915 New York State Constitutional Convention, serving as chairman of the Committee on Governors and other State Officers. He was in the New York National Guard, serving as private, corporal, and sergeant in the 23rd Regiment, 2nd Lieutenant in the 12th Regiment, and 1st Lieutenant and Captain in the 14th Regiment. In the 1918 United States House of Representatives election, he was the Republican candidate for New York's 17th congressional district. He lost the election to Herbert Pell.