After the capture of New Brunswick by the British during the war, De Witt fled to New York City where he joined the Revolutionary Army.

In June 1778, having been trained as a surveyor by James Clinton, the husband of De Witt's Aunt Mary, De Witt was appointed as assistant to the Geographer and Surveyor of the Army, Colonel Robert Erskine, and contributed to a number of historically significant maps.

[2] After the American Revolutionary War, De Witt attempted, but failed, to get the Continental Congress interested in a national mapping project.

Washington wrote to Thomas Jefferson about De Witt "I can assure you, he is extremely modest, sensible, sober, discreet, and deserving of favors.

Frustrated by opposition from landowners, who wanted to determine for themselves where streets would go as they developed their properties, and interference from various political factions, the Council had called on the state for assistance.

The Commission was given "exclusive power to lay out streets, roads, and public squares, of such width, extent, and direction, as to them shall seem most conducive to public good, and to shut up, or direct to be shut up, any streets or parts thereof which have been heretofore laid out... [but] not accepted by the Common Council.

[12] Ironically, considering the massive effect on Manhattan of the Commissioners' Plan, De Witt himself did not much like New York City.

[3] De Witt was almost six-foot (1.8 m) tall, and was described by his son as having "a noble, serious face, resembling in some respects that of Genl Washington."

De Witt died in Ithaca after having caught a very bad cold while traveling to his various properties in upstate New York.

[2] On May 25, 2010 the Smithsonian Institution National Museum of American History exhibited the oldest surviving Anglo-American star map, hand-drawn in 1780 by Simeon De Witt, in its Albert H. Small Documents Gallery.

Drawing such a map, as De Witt himself later said, fostered an appreciation of "the ever shifting scenery of the skies and all the gorgeous drapery of heaven."