 Platt was born on June 30, 1769 in Poughkeepsie, Province of New York, in what was then British America.He was the son of politician and lawyer Zephaniah Platt (1735–1807), who founded Plattsburgh, New York, and his second wife, Mary Van Wyck Platt (1742–1809).Among his siblings was New York State Treasurer Charles Z. Platt.