Answer:
That depends on your view of the social contract. Some philosophers, like David Hume, might say there was no time when EVERYBODY consented to a government so there were none!
In a more practical sense, the first social contract would probably be the mayflower compact in the 1640s. this was pretty close to a legitimate contract because every one who wanted to be protected by it had to sign it. (at least every white christian male).
If you mean the first governments assembled by Americans in America, then the first American social contracts might also be attributed to the first state constitutions. the first of which was implemented in 1775.