The USA has sent diplomatic representatives to China as far back as 1844. At first, they were envoys and later styled ‘Ambassadors’. The Communist Revolution of 1949 really upended the diplomatic world. I have only spoken with two former U.S. Ambassadors to China: Jon Huntsman, Jr (22 times) and Terry Branstad (49 times); but solely on domestic matters. That was while they were incumbent governors of Utah and Iowa, respectively.