George Washington at Princeton was bought by art dealer CL Prickett and set a world record for the sale of an American portrait, Christie's said.
The president is pictured during the US War of Independence.
Peale, one of the revolutionary era's most revered artists, signed and dated the artwork in 1779.
It is the only one privately owned and Christie's had expected it to fetch between $10m and $15m.
Washington at war
The 8ft (2.4m) portrait celebrates victories at Trenton and Princeton and was sent to Spain by the artist soon after its completion.
It was intended to promote the American cause aboard and coincided with Spain's formal entry into the conflict.
During the war Washington was commander of the Continental Army, which, with the aid of the French, forced the British to surrender in 1781.
Ten years later, in 1789, he became the first American president.
The sale more than doubled the previous record for an auctioned American - Sotheby's sold a half-length portrait of Washington in November for $8.1m ($4.6m).
George Washington at Princeton was sold as part of a collection of US furniture and arts from Natalie Knowlton Blair, who bought the painting with her banker husband J Insley Blair in 1919.
She died in 1951 and the collection was being sold by her grandson.
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