Pinckney initially declined appointment to a federal position, but in 1792 he agreed to serve as President George Washington's ambassador to Britain. As Pinckney was unable to get the British to reach an agreement on various issues, including the practice of impressment or the evacuation of British forts in American territory, Washington dispatched John Jay as a special envoy to Britain. Pinckney helped Jay conclude the Jay Treaty, which addressed some issues between the U.S. and Britain but proved divisive in the United States. In 1795, while he continued to serve as the ambassador to Britain, Pinckney was sent to Spain to negotiate a treaty regarding boundaries and U.S. navigation on the Mississippi River. In the resulting Treaty of San Lorenzo, Spain agreed to allow Americans to export goods through the Mississippi River.