He was appointed by President James Monroe as America's first United States Minister Plenipotentiary to the new republic of Chile beginning on January 27, 1823. When he called on Commissioner Isaac Hull, whose wife was at her family home saying goodbye before the Hulls sailed to Lima, Allen was making arrangements for passage on the frigate "United States." He met Mrs. Hull's sister Elizabeth, one of the "Seven Lovely Hart Sisters" of Saybrook, and in two weeks they were married and en route to South America. Ann Hart Hull's sister, Jeannette, sailed with the Hulls, and was said to have been proposed to by widower Simon Bolivar after his triumphal entry into Lima in Feb. 1824, and that she refused after learning of his fiery and possessive Ecuadorian mistress, Manuelita Saenz. Allen continued in Chile as minister until July 31, 1827.