During the Civil War, Platt joined the Union Army as first sergeant of the Third Regiment, Vermont Volunteer Infantry.
He served from January 26, 1870, to March 3, 1875, including as chairman of the Committee on Public Buildings and Grounds (Forty-third Congress).
In 1874, conservative politicians won throughout Virginia after the Panic of 1873 led to financial depression, and Platt lost his re-election bid.
Democrat (and ex-Confederate congressman) John Goode won 49.43% of the vote, easily defeating both Platt and Independent Republican Robert Norton, and represented the district in the Forty-fourth Congress.
Platt moved to New York in 1876 and engaged in the manufacture of oil products, as well as continued his medical career as director of the Mineola Children's Home.