[6] On September 23, 1780, British agent Major John André was captured by Continental Army troops in North America; he was hanged as a spy on October 2, 1780.

After news reached Great Britain, outrage flared and Trumbull was arrested for treason, since he was known to be an officer in the Continental Army and of similar rank to André.

Arriving in late January 1782, he found employment with his brother David as a commissary agent for the army stationed at New Windsor, New York, during the winter of 1782 and 1783.

[10][11] In 1784, following Britain's recognition of United States' sovereignty and independence, Trumbull returned to London to complete his apprenticeship with West.

His first major work, The Deputation from the Senate Presenting to Cincinnatus the Command of the Roman Armies, was accepted and displayed by the Royal Academy of Arts in that year.

In July 1786, Trumbull traveled to Paris, where he made portrait sketches of French officers, including Surrender of Lord Cornwallis.

With assistance from Thomas Jefferson, who was then serving in Paris as the American minister to France, Trumbull began the early composition of the Declaration of Independence.

While visiting with each signer or their family, Trumbull was always seeking funding and used the occasion to sell subscriptions to engravings that he produced from his paintings of the American Revolution.

In 1794, Trumbull acted as secretary to John Jay in London during the negotiation of the treaty with Great Britain, which largely settled the boundary with Canada and began cotton export to the country.

In 1796, he was appointed by the commissioners sent by the two countries as the fifth member of a commission charged with carrying out the seventh article of the Jay Treaty,[4] which mediated claims by American and British merchants and the opposing government stemming from actions that occurred during the war.

Shortly after the end of Trumbull's service on this commission, he traveled to Stuttgart to pick up the completed engraving of the Battle of Bunker's Hill.