The double world champion made history for his country by edging out Kazakh Nariman Kurbanov by a wafer-thin 0.100 points, with Stephen Nedoroscik of the United States in bronze.
Britain's most decorated gymnast Max Whitlock was deprived a perfect send-off to his decorated Olympic career, finishing fourth.
McClenaghan crumpled with emotion after his faultless 60-second performance crammed with Russian circles, scissors and handstands, finishing off with a foot-perfect dismount.
"There are so many thoughts going through my head," said the 25-year-old from Northern Ireland.
"I've accomplished a lifelong dream here. I can't believe it's happened. I always felt like it was going to happen, I just wasn't sure when."
McClenaghan arrived at the eight-man final as favourite after heading qualifying and with his two world titles.
But he also had to contend with heaps of pressure after falling off the apparatus at the Covid-delayed Tokyo Games three years ago to trail in seventh.
He wasn't about to let this second shot at writing his name in Irish sporting folklore go awry though.
With a score of 15.533 he wept tears of joy on the top of the podium as he listened to "Amhran na bhFiann" - the Irish national anthem - with green, white and orange tricolours being waved proudly from the stands by his legion of supporters at the Bercy Arena.
'I feel amazing'
Kurbanov had been the first to perform and he set a high bar with a score of 15.433.
Next up was Whitlock, seeking an unprecedented fourth medal on the same apparatus after golds at the last two Games in Tokyo and Rio, and bronze at London 2012.
And the 31-year-old's slick Olympic swansong was enough to leave him sitting third behind McClenaghan and Kurbanov until Nedoroscik took his place on the podium by 0.100pts.
Nedoroscik is the US gymnastics team's one-trick pony - the pommel horse his sole speciality.
His father bought him an old one from the 1980s at auction when he was a youngster and from that moment he was hooked.
"When you only do one event you don't get to test the others. You just go out there and swing. But I've been doing this for eight years now as a specialist, so really tested the waters. I feel amazing," said the man from Massachusetts.
He had posted the same score as McClenaghan in qualifying, meaning Ireland's third gold medallist of the 2024 Games had a tense wait to see if his American title rival would gatecrash his party.
He said he purposely hadn't watched those performing after him, but that didn't stop the nerves.
"I was nervous. I'm nervous in any competition. I'm nervous in every training session. I feel like the pressure I put on my shoulders pushes me every day. That's what resulted in this."
And what does the immediate future hold in store for the new prince of the pommel horse?
"I want to see my friends and family. Maybe live a normal life for a couple of months, but I'm looking forward to the comeback as well," he said.
