Born in Liverpool, Morris migrated to the United States in his teens, quickly becoming a partner in a successful shipping firm based in Philadelphia. In the aftermath of the French and Indian War, Morris joined with other merchants in opposing British tax policies such as the 1765 Stamp Act. After the outbreak of the American Revolutionary War, he helped procure arms and ammunition for the revolutionary cause, and in late 1775 he was chosen as a delegate to the Second Continental Congress. As a member of Congress, he served on the Secret Committee of Trade, which handled the procurement of supplies, the Committee of Correspondence, which handled foreign affairs, and the Marine Committee, which oversaw the Continental Navy. Morris was a leading member of Congress until he resigned in 1778. Out of office, Morris refocused on his merchant career and won election to the Pennsylvania Assembly, where he became a leader of the "Republican" faction that sought alterations to the Pennsylvania Constitution.