Platt was elected as a Republican to the Sixty-third and to the three succeeding Congresses and held office from March 4, 1913 to June 7, 1920, when he resigned to accept appointment by President Woodrow Wilson to the Federal Reserve Board. While in the House of Representatives, he was chairman of the Committee on Banking and Currency (Sixty-sixth Congress). Platt became vice governor of the Federal Reserve Board in August 1920 and served until 1930 when he resigned. He returned to Poughkeepsie and engaged in an extensive banking business; he died in Chazy, New York while on a visit in 1939; interment was in the Poughkeepsie Rural Cemetery.
