John Jay had been serving as Secretary of Foreign Affairs, a position he'd held for five years, when in 1789 President George Washington asked him to carry on with effectively the same duties in the newly created office of the Secretary of State. When Jay declined the offer, Washington had to find a new job for his old friend.
On September 24, 1789, Washington signed the Judiciary Act of 1789, which established that the Supreme Court be comprised of one Chief Justice and five Associate Justices. That was the very same day that Washington nominated Jay to be the very first Chief Justice of the Supreme Court. And so, on October 19, 1789, John Jay was sworn in as the nation's first Chief Justice of the Supreme Court, while simultaneously serving as Acting Secretary of State--that's quite a bit of juice.
The first session of the Supreme Court was convened February 2, 1790, in New York City, and lasted all of eight days as the newly minted sextet had no cases to hear. Instead they spent the time establishing rules. Jay led the Court for nearly six years, during which time they heard only a handful of cases and issued only one major decision, Chisholm v. Georgia, which allowed for individual states to be sued by other states, but was later superseded in 1795 by the 11th Amendment. Good job, fellas.