The first one where it was known at the time he usurped the office. Vice President Chester Arthur's father was not a citizen at the time of his birth, although Chester was born in the US, and his father did eventually become naturalized when Chester was about 14. Arthur became President upon the assassination of President Garfield in 1881.
But in this case we do know.
I have been trying to establish the date when the fightthesmears.org site posted Obama’s admission that his father was a Kenyan and he had dual citizenship at birth. I wonder if it coincides with something in the media about McCain being approved by the Laehy committee and the non-binding resolution vote in the Seante?