In the fall of 1775, he arrived in Holland to seek supplies for the rebellious American colonies. When this mission was thwarted by British diplomacy, Conyngham went to France and, in March 1777, obtained a commission as Captain in the Continental Navy. Given command of the lugger Surprise (or Surprize), he began a fruitful commerce raiding career by capturing two British merchant vessels in the North Sea early in May and taking them to Dunkirk.
Although the French soon returned his prizes and seized Surprise, Conyngham was made captain of the cutter Revenge, in which he was to capture or destroy more than sixty enemy merchantmen and a few privateers during a remarkably productive cruise through the waters around the British Isles, off Spain and in the Caribbean Sea between July 1777 and February 1779. After returning to the United States, he undertook another voyage, but was captured and imprisoned in England. Captain Conyngham managed to escape after several attempts and, after reaching France, was briefly an officer of the Continental frigate Alliance. Recaptured again while sailing back to the Colonies in mid-March 1780, he remained a prisoner until exchanged in June 1781.
Following the end of the Revolutionary War, Conyngham resumed his commercial shipping career. He also began a long and frustrating effort to gain Congressional recognition for his contributions to the struggle for American independence. Conyngham’s later martial involvement included command of an armed brig in the Quasi-War with France and assisting with the defense of Philadelphia during the War of 1812.
He died at Philadelphia on 27 November 1819.