In 1903, Prentice was elected to the New York State Assembly as a Republican, representing the New York County 25th District. He served in the Assembly in 1904, 1905, 1906, and 1907. While in the Assembly, he was a member of the Armstrong Committee. He was a delegate to the 1908 and 1912 Republican National Conventions. He was Deputy Attorney General of New York from 1909 to 1910. was a member of the New York Republican State Committee, and in October 1910 he was elected Chairman of the Committee after Theodore Roosevelt ousted the Party's Old Guard. He resigned as chairman after Henry L. Stimson lost the governor's election to John Alden Dix.