Tied for 17th starting the final round, nine strokes behind Mito Pereira, the four-time major champion produced an ultimately fleeting final round charge that did little to take the sting out of another nearly week.
Four birdies in a row from the second raised hopes that he could post a formidable target and beat John Mehaffey’s record PGA comeback of 1978, when he came from seven strokes behind Tom Watson with a 66 at Oakmont to force a playoff and beat Watson and Jerry Pate at the second extra hole.
But he played the remaining holes in one-over par, never holing a putt outside four feet until he brushed in an 11 footer for a closing par and a two-under 68 that left him tied for eighth in the clubhouse on two-under par.
After opening with a 65, McIlroy looked set to mount a sustained challenge for that elusive fifth major, nearly eight years after hoisting the Wanamaker Trophy for the second time at Valhalla.
But after a ho-hum 71 in the best of the conditions on Friday, followed by a bitterly disappointing 74 on Saturday, he left himself an impossible nine-shot deficit on the final day.
Shane Lowry, whose closing 69 left him tied for 23rd on two-over, called on the keyboard warriors to be more understanding of the Co Down man, who did not stop to talk after his round.
"The armchair golfers don't realise how hard it is out here," Lowry said. "It's not as easy as shooting 65 in the first round.
"I saw a quote from him from last week, I don't know when it was from, where was he hasn't won a major since 2014, but he has pretty much done everything else you have to do in the world of golf.
"So it's not like he's in a slump. It's almost like you're a victim of your own success. People expected him to win 10 majors when he won those four and it just hasn't worked out like that. But he's only 33 now and he still got a long way to go in his career."
McIlroy was tied fourth for greens hit in regulation as the final round concluded but he was outside the top 40 for putting and one wonders what Dr Bob Rotella will be telling him after this latest near miss.
After opening with a solid par, he ripped a 193-yard approach to 15 feet at the second and rolled in the putt, then blasted a 329-yard tee shot down the third and punched a short iron approach to seven feet and made that to move into the top ten on two-under.
When he drove into the deep rough left of the 368 yard fourth, a birdie looked unlikely but he found the green and then made a 25 footer for his third birdie in a row to tie for sixth with Seamus Power on three-under, six shots off the lead.
With the par-five fifth ahead, he launched a 357-yard tee shot but while his three iron from 256 yards found a deep bunker on the right, he splashed out to four and a half feet below the cup and brushed in the putt to give himself hope of posting a round in the mid to low sixties that might give the leaders food for thought.
Another birdie at the difficult, par-three sixth would have created a sensation but he missed the green right and semi-duffed his pitch, coming up 20 feet short before missing the putt.
That halted his momentum and he didn’t make a birdie after that.
He had chances too but missed from 11 feet at the ninth, 15 feet at the 12th, 10 feet at the 13th and 13 feet at the 15th before pitching through the green at the driveable, 302-yard 17th and walking off with a bogey after missing a 12 footer for par.
At the 18th he came up short of the green but pitched to 11 feet and made the putt for par and a two-under 68 that left him tied for eighth in the clubhouse.
As for Lowry, he knows he just has to remain patient and keep playing the golf he's playing to have more chances to add to this major haul.
The Offaly man made 17 pars and one birdie in a one-under 69 to finish inside the top 25 without ever hitting top gear.
He might be winless since he captured The Open in 2019, but insists he must simply remain patient and he will have further chances to lift another Grand Slam trophy.
"Like I said to Bo going down the last, I feel like I'm two or three shots away from having a good week," Lowry said. "But I'm disappointed because I feel like I could've done better this week.
"It is still another decent performance there's no real bad things to take away from that. Yes, I got up and down a good few times when I need to do, but I did miss quite a few chances out there to shoot a decent score.
"But it is another decent performance and I'm going to finish in the top 30 somewhere. It'll be a few FedEx Cup and Race to Dubai points. It is what it is and I'm happy enough for the week. There are no real things I need to go away and work on and I'm looking forward to the next big one.”
After making par saves at the first three holes, Lowry finally rolled in a 15 footer for his lone birdie of the day at the 11th.
He had further chances coming in, but while an 18 footer slipped past the cut at the 18th, he gave the two-foot return putt extra attention to finish with a clean card.
"I was eager not to bogey the last," he said. "I was conscious of that and to be honest, I thought if I could roll that in, I would have finished somewhere inside the top 20, which would have been nice. It is what it is."
With two top-three finishes in his last seven starts, Lowry is playing the most consistent golf of his career. But he's not taking it for granted as he prepares for the Memorial Tournament and the RBC Canadian Open before heading for the US Open at Brookline.
"You still need to knock off performances like this and you still need to learn," he said. "The thing for me is I felt like the last couple of months I've been up there every week and today felt a little flat of times.
"I felt like I was struggling to get it going. You could see I wasn't far away, and if I made three birdies in a row, you have a chance of a really nice week.
"I just still need to have these weeks and remain patient and keep playing good golf in these tournaments, and if I keep doing that I will be OK.
"I'm not taking anything for granted. I know how fickle this game is. I know that I need to take the rough with the smooth and just kind of be myself and go about my business and not try to force these good weeks because you just don't know what's around the corner.”
Tied ninth in the US Open at Chambers Bay in 2015 and tied second at Oakmont the following year, when he had a four-shot lead after 54 holes, he knows the pressure will be on him in Irish-made Boston this year.
"I've heard I'm going to be quite popular there," he said with a grin. "So hopefully, I can bring some good golf there and give them something to shout about.”