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Boris Johnson was also born an American citizen, but renounced it before coming prime minister. Not technically head of state (the queen was), but close enough.

If my memory doesn’t fail, there has also been an American adventurer called William Walker who doubled as president of Nicaragua back in the 19th century

Just read a little about him, seems like a very weird guy who basically invaded Nicaragua and Bolivia on his own and declared himself president. It looks like he never had full control over the country and wasn't recognized as the valid president by most other nations, but he does seem to be the next closest.

Golda Meir was the Prime Minister (head of government) of Israel after having naturalized in the US when she was 19.

Looks like she gave up her US citizenship when she moved, as did Boris Johnson who was also mentioned. So I haven't seen anyone who retained citizenship and was a recognized head of state.

This is some excellent trivia. Thanks!