Promoted to lieutenant colonel in the Royal Corps of Engineers, Duportail was secretly sent to America in March 1777 to serve in Washington's Continental Army under an agreement between Benjamin Franklin and the government of King Louis XVI of France.

Duportail helped Washington evolve the primarily defensive military strategy that wore down the British Army, and participated in fortifications planning from Boston, Massachusetts to Charleston, South Carolina, where he was captured following the surrender of the city in May 1780.

Subsequently exchanged, he also directed the construction of siege works at the Battle of Yorktown, site of the decisive Franco-American victory of the Revolutionary War.

He served as France's minister of war from November 16, 1790, through December 7, 1791, during the beginning of the French Revolution and promoted military reforms.

Forced into hiding by radical Jacobins, he escaped to America and bought a farm near Valley Forge, Pennsylvania.