Born in Williamsburg, Virginia, Blair was a member of a prominent Virginia family. He served as mayor of Williamsburg, Virginia from 1751 to 1752. His father served on the Virginia Council and was for a time acting Royal governor. His granduncle, James Blair, was founder and first president of the College of William and Mary. Blair attended William and Mary, receiving an . in 1754. In 1755 he went to London to study law at the Middle Temple. Returning home to practice law, he was quickly thrust into public life, beginning his public career shortly after the close of the French and Indian War with his election to the seat reserved for the College of William and Mary in the House of Burgesses (1766-70). He went on to become clerk of the Royal Governor's Council, the upper house of the colonial legislature (1770-75).
As for Virginia, which only ratified the Constitution at its convention on June 25, Hamilton writes in a letter to Madison that the collected edition of The Federalist had been sent to Virginia; Furtwangler presumes that it was to act as a "debater's handbook for the convention there," though he claims that this indirect influence would be a "dubious distinction."  Probably of greater importance to the Virginia debate, in any case, were George Washington's support for the proposed Constitution and the presence of Madison and Edmund Randolph , the governor, at the convention arguing for ratification.