Early in 1777 he was sent to buy gunpowder for the American army in Holland but became stranded there and applied to the American commissioners in France for employment. He was appointed captain of the armed lugger Surprise and created a sensation by capturing a British mail packet and taking it to the French port of Dunkerque for sale as a prize of war. Since France and Great Britain were not then at war, the French authorities returned the ship to the British with apologies and confiscated Conyngham's commission. He was then given command of the 10-gun cutter Revenge and sank or captured more than 60 British merchant ships in the next two years. Conyngham was captured by a British warship in April 1779 and sent to England in irons for trial as a pirate, but escaped and made his way to Holland. Most of his career after that was anticlimactic, but he did make a cruise with John Paul Jones aboard the frigate Alliance. Because Congress failed to confirm the commission issued to Conyngham by the American commissioners in France, he was never paid for his services during the Revolutionary War. He died at Philadelphia, Pa., on Nov. 27, 1819.