Interesting answers. On the mainland of North America, I suppose George III was an "American" king, which would mean the last was Louis XVI of France, who still ruled Louisiana until he was deposed in the French Revolution. If South America is included that would be Pedro II, Emperor of Brazil to 1891. Or Queen Elizabeth II of the UK, since Belize did not become independent until 1966. Or Queen Beatrice of the Netherlands, since Suriname did not achieve independence until 1975. The Netherlands in fact still owns territories in the Caribbean.
If we mean Native American kings, that would be Moctezuma of the Aztecs in North America and Manco Inca Yupanqui in the South, brother of the deposed Atahualpa of the Incas.
Of course, we probably should not include Emperor Norton I, "Emperor of these United States and Protector of Mexico" from 1859 until his death in 1880. You really should look him up, he was far more interesting than Elvis or Michael Jackson.
By the way, all American presidents have been related by either blood or marriage. Some have been maternal grandsons of previous presidents or married to a granddaughter; some have been nephews or married to nieces; some have been cousins or married to cousins. But all have some sort of familial connection. While Roosevelt and Roosevelt were 4th cousins, Madison and Taylor were 3rd cousins; Taylor's son-in-law was Jefferson Davis. Jackson and Tyler were both descended from a 17th century Virginia planter named Armistead. Benjamin and W.H. Harrison were grandfather and grandson; the latter Harrison and Jefferson were cousins; the Roosevelts, the Adamses and Monroe were all related through one of Harvard's founders; Cleveland, Garfield and Pierce were cousins; Coolidge was related to Jackson through grandchildren of Jefferson; Truman was descended from Tyler; LBJ was descended from a Harrison who was the grandfather of William Henry Harrison. Nixon through marriage was connected to Monroe and Taylor (as well as Eisenhower, who hated Nixon), etc. Hmm.... maybe the most recent American king is Barak Obama.