Janet Jagan was born in the U.S. and became president of Guyana, however she had lost her U.S. citizenship by the time of her election.

Here's a NYT profile shortly before she won the election: http://www.nytimes.com/1997/12/14/world/a-guyana-favorite-us-born-grandmother.html?mcubz=1

The former King of Thailand was also born in the United States, however Thailand doesn't recognize dual citizenship after 18.

edit (ninja?): Eamon de Valera was also born in the United States, which may have spared his life in the Easter Rising. He later became Prime Minister and then President of Ireland.

Eamon De Valera was born in New York in 1882. He was sent back to his mother's home in Ireland when he was about 2 years old and raised there. He was a commander in the 1916 Rising against British rule and also a leader during the Irish War of Independence.

Later he became head of the Irish government, Taoiseach, a post he held for over three decades. Then President of Ireland from 1959 to 1973.

Not too surprising given its origin as an African colony of free-born American blacks several of the early presidents of Liberia were born (and raised) in the USA. Examples include Joseph Jenkins Roberts, Stephen Allen Benson, Daniel Bashiel Warner, James Spriggs Payne, Edward James Roye, James Skivring Smith, Anthony W. Gardiner, Alfred Francis Russell, William D. Coleman, and Garretson W. Gibson. However since African-Americans weren't given American citizenship until the 14th Amendment in 1868 and at that time those Americo-Liberians were no longer subjects of the USA but instead subjects of a recognized foreign power (i.e. Liberia) none of these American-born Liberian presidents were ever officially "American citizens".

Edit: Also before the Republic of Texas was annexed by the US all its presidents (such as Sam Houston) were American citizens.