The answer is:

Benjamin Franklin was the first U.S. diplomat. He was appointed on September 26, 1776 as part of a commission charged with gaining French support for American independence. He was appointed Minister to France on September 14, 1778 and presented his credentials on March 23, 1779, becoming the first American diplomat to be received by a foreign government. Franklin was one of three Commissioners who negotiated the peace treaty with Great Britain, and continued to serve in France until May 17, 1785. [source: U.S. State Department]