The breaststroke gold was his third at the La Defense Arena but Marchand rewrote the script on what was previously thought possible by becoming the first to medal in both of the demanding disciplines.
Even U.S. great Michael Phelps, whose coach Bob Bowman now works with the Frenchman, did not achieve that with his mighty wingspan.
With the crowd chanting his name at every stroke - "Leon, Leon" echoing around the rafters - Marchand led every metre of the breaststroke before touching out in two minutes 05.85 seconds.
The world record, set by China's Qin Haiyang last year, stands at 2:05.48 and Marchand was set to beat that up to the 150 metre mark.
Australia's Zac Stubblety-Cook, the 2021 champion in Tokyo, had to settle for silver while Caspar Corbeau of the Netherlands took the bronze.
"It was pretty crazy to have the chance to be in the final in both races," said Marchand. "And then I just loved it, it was huge.
"I've fulfilled a lot of dreams since I've been here. Doing this double was also something I felt capable of, but to really do it in the real world is something else."
The youngster, who lit the touchpaper when he won the Individual Medley on Sunday, sent sound levels off the scale from the moment he stepped onto the pool deck.
In the butterfly race he beat Hungary's world record holder and reigning champion Kristof Milak into second place, with Canada's Ilya Kharun finishing third.
Unlike in the later breaststroke, Marchand was behind until the final length when he overpowered Milak and surged past to win.
He returned for the podium ceremony, saluting the cheering crowd as the "Marseillaise" national anthem sounded, and then made a quick exit.
Such was the sense of anticipation ahead of the later event that the poolside volunteers removing the athletes' clothing boxes left more hurriedly than usual, sprinting even, to catch the race on television.
All over town crowds waited and watched history unfold, the bars spilling over.
Spectators at the table tennis venue broke into the anthem after Marchand's first gold of the night.
To win both titles was a simply astonishing achievement. To do it at a single Games even more remarkable. To take double gold on the same night, with a medal ceremony in between, truly the stuff of fiction.
"I had a lot of doubts, because everyone was telling me it wasn't possible," said Marchand.
"I had the shivers all through the race, and at the end I really tried not to breathe, to hit the wall as quickly as possible, and then I see I'm first, it was a pretty crazy emotion," he said
"I really enjoyed every moment of these two finals. It was really amazing for me to swim both. I'm really proud of everything."
