Pete Stark was the first openly atheist member of Congress in America
http://en.wikipedia.org/wiki/Pete_Stark
He announced his atheism in 2007
Octavious
It was rather difficult to be an out and out atheist in the 18th Century - there simply wasn't the intellectual framework for it.
It was a time before Darwin, before Cosmology before the age of the Eath was known, before we even knew Dinosaurs even existed.
Most of the people then who would today be atheists were Deists.
They rejected established notions of religion, but in the lack of alternative explanation went for a rather vague picture of a creator - quite well summed up as Spinoza's God.
At this time this is about as far as you get. It was dangerous to be an atheist - some like Jean Messlier left documents for after their death but you have to wait really for the 19th Century to find open atheists.