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.

There is always Elvis...this discussion is quite entertaining and enlightening.  I wouldn't completely discount the King of Rock and Roll, or Michael Jackson, the King of Pop.  Of course, they were self-proclaimed Kings, and neither was part of a royal family or dynasty as we typically think of Kings and Queens.

The previous posts are all excellent, especially the one concerning Queen Liliuokalani of Hawaii. I would never have thought of that one. Since there has never been a true King of America, I would have to go with King George III of Great Britain, since he was the last ruler of the American colonies before the nation declared its independence. Of course, if Rufus King had been elected President when he ran in 1816, he would have become both a president and King.

That's because you didn't go to HS in Hawaii... LOL

The Founding Fathers offered to make George Washington king, but he was not agreeable to becoming a monarch since the Revolutionary War was fought against King George III--the king of England when the U.S. was still the 13 British colonies--and a democratic republic was a concept much more popular to the people.

In the 1970s, journalists and pundits often called Richard Nixon  "King Richard" because of statements he made that were autocratic and demonstrated his desire for power.  For one state occasion, he even had trumpeters dressed in older English-style clothes blowing the long horn of "ye ole years" for--to allude to a title--"The Man Who Would Be King."

Of course,the Watergate Scandal underscored Nixon's power hunger as he acted as though he were above the law. But, then, he was certainly not. To ensure against meglomania after Nixon's "reign," certain restrictions were put upon the executive branch with rulings by Congress.

Depends on what you mean by "American."  When it comes to places that are part of the United States today, the last monarch was Queen Liliuokalani of Hawaii.  She was deposed by forces that wanted to overthrow the Hawaiian monarchy and force Hawaii to become a territory of the US.