The two now share the four crown jewels of the sport and all the talk is of a rivalry that will fire up a golfing world struggling to cope with the fading legend that is Tiger Woods.
Both players say they welcome the prospect of going head-to-head down the back nine of the majors over the next decade or so, although Spieth, the younger by five years, believes he still has something to prove.
"It's kind of cool, I think, to have two players holding the four majors and Rickie [Fowler] having the fifth [Players Championship]. It's awesome that the game is in young hands," the Texan said. "I don't think there is much of a rivalry. I've said that from the beginning. Rory has four majors and dozens of wins and I'm just starting out.
"Again, I'm certainly quite a bit younger than he is. I'm just happy to have this and to be chasing that number one spot that he holds. So I'm certainly focused on that."
His win at Chambers Bay has closed the gap on McIlroy at the top of the world rankings to just 1.72 points after trailing him by 5.30 points at the end of last year.
They will next clash at the year's third major - the British Open at St Andrews in mid-July where McIlroy will have the advantage of knowing the fabled course far better than Spieth, who has played just a single round there.
Then it is on to the PGA Championship in August, where McIlroy tied for third place in the 2010 PGA Championship.
But Spieth tellingly believes he still has room for improvement "in all aspects of my game" and it may be time now for McIlroy to step out of his comfort zone and take another step forward.
Ryder Cup captain Paul McGinley, for one, believes McIlroy needs to respond to the challenge posed by Spieth.
"All the plaudits will go to Jordan Spieth and rightly so, with back-to-back majors and a massive career ahead of him," he said in his column for Sky Sports.
"When someone wins their first major like Jordan did at the Masters, you expect added pressure and expectation put on their shoulders, but he just went and won the next one. He's got something special." AAP