He graduated from Dartmouth College in 1804, studied law with Chief Justice Theophilus Parsons, and became an eminent lawyer.
During the War of 1812, he commanded a regiment of militia on the coast defences.
Knapp was elected a member of the American Antiquarian Society in 1814.
In 1826 he established the National Republican, which failed two years later, and he returned to practicing law in New York City.
[3] His works, which are chiefly biographical, include: He edited "The Library of American History" (New York, 1837).