Enlisted in the "Horse Guard" in 1820. He served as mayor 1826–29. Pratt was elected major of the First Regiment of Cavalry in 1834. He served as colonel in 1836, brigadier general 1837–39 and a major general 1839–46.
Pratt then served as adjutant general in 1846, retiring from mercantile pursuits and settled in Rocky Hill, Connecticut.
He was again a member of the State house of representatives in 1857 and 1862.
Pratt was elected as a Democrat to the Thirty-third Congress (March 4, 1853 – March 3, 1855).
He was an unsuccessful candidate for reelection in 1854 to the Thirty-fourth Congress. Pratt was an unsuccessful candidate for election as governor in 1858 and 1859.
Pratt served as member of the peace convention of 1861 held in Washington, D.C., in an effort to devise means to prevent the impending war.