Platt's political involvement began at the Republican Party's inception; he made his first appearance in politics in 1856 in the campaign of the party's first presidential candidate, John C. Fremont. Running as a Republican, he was elected clerk of Tioga County, serving from 1859 to 1861. He was elected as a Republican to the Forty-third United States Congress and the Forty-fourth United States Congress, serving from March 4, 1873 to March 3, 1877. His influence on statewide politics began on his return from Congress in 1877, when he aligned with the "Stalwart" faction led by US Senator Roscoe Conkling at the party's state convention, and against the "Half-Breed" faction loyal to President Rutherford B. Hayes.