In 1780, he was one of three men who captured British Major John André, who was convicted and executed as a spy for conspiring with treasonous Continental general and commandant of West Point Benedict Arnold.

[5] A marble and granite monument was erected at his grave on 11 June 1829, bears the single emphatic word "FIDELITY", followed by this epitaph, On the 23rd of September 1780, Isaac Van Wart, accompanied by John Paulding and David Williams, all Farmers of the County of Westchester, intercepted Major André, on his return from the American Lines in the character of a Spy, and notwithstanding the large bribes offered them for his release, nobly disdaining to sacrifice their Country for Gold, Secured and carried him to the Commanding Officer of the district, whereby the dangerous and traitorous Conspiracy of Arnold was brought to light; the insidious designs of the enemy baffled; the American Army saved; and our beloved country now free and Independent, rescued from most imminent peril.The three militiamen were highly celebrated in their lifetimes: commemorations large and small abound in Westchester, and can be found in many disparate parts of the early United States.

André at his trial had insisted the men were mere brigands; sympathy for him remained in some more aristocratic American quarters (and grew to legend in England, where he was buried in Westminster Abbey).

Giving voice to this sympathy, Representative Benjamin Tallmadge of Connecticut persuaded Congress to deny the men a requested pension increase in 1817, publicly assailing their credibility and motivations.

Some modern scholars have interpreted the episode as a major event in early American cultural development, representing the apotheosis of the common man in the new democratic society.

[6] Van Wart and his companions are honored on the monument erected at the site of the capture in Tarrytown, dedicated on June 11, 1829, by the Revolutionary general and congressman Aaron Ward of nearby Ossining.