Marchand's triumph seemed almost pre-destined in a stadium festooned with tricolore flags. But the magic did not quite rub off for Briton Adam Peaty, whose historic bid for a third successive breaststroke gold was foiled by Italian winner Nicolo Martinenghi.
There was also a surprise in the women's 100 metres butterfly as American Torri Huske ambushed her world record-holding teammate Gretchen Walsh, taking the gold by a fingertip.
The night belonged to Marchand, however, as he shrugged off the pressure of being the face of the Games with a race energised by ear-splitting support from the terraces.
Ultimately, there was no contest and barely another swimmer in the frame as the 22-year-old nicknamed the "French Michael Phelps" touched the wall in four minutes and 2.95 seconds, nearly six seconds in front of Japan's silver medallist Tomoyuki Matsushita.
American Carson Foster took the bronze.
With hands on hearts, fans belted out "La Marseillaise", the French national anthem, with Marchand as he beamed atop the podium, having become the country's first medley gold medallist.
Though in a working capacity with broadcaster NBC, Phelps himself was a fitting presence at La Defense Arena along with Bob Bowman, the coach who turned him into a gold medal machine and has now worked his magic on Marchand.
Phelps remains the only man to win three or more swimming gold medals in the same event following Peaty's agonising near-miss in the 100 metres breaststroke.
Three years after taking bronze behind Peaty at the Tokyo Games, Martinenghi won his first Olympic gold with a time of 59.03 seconds, 0.02 seconds ahead of the Briton and American Nic Fink.
Peaty and Fink both earned silvers, with no bronze awarded.
"I am not crying because I have come second," said Peaty, whose Olympic tally now stands at six medals, three gold and three silver.
"I am crying because of how much it took to get here," he told the BBC.
"In my heart I have won. These are happy tears."
Martinenghi's win made it three-out-of-three first-time Olympic gold medallists in the pool on Sunday, following former world champion Huske's tight win over Walsh.
The 2022 world champion touched the wall in 55.59 seconds to secure the U.S. team's first individual swimming gold of the 2024 Games, with Walsh taking silver in 55.63 after leading at the turn on world record pace.
Tokyo runner-up Zhang Yufei took her second bronze of the Paris Games a day after she helped China finish third in the women's 4x100m freestyle relay.
Zhang had said on day one that she was worried that a doping storm would cause her rivals to view her differently, but on Sunday she smiled on the medal podium and linked arms with Huske.
"After all, we are old friends and old rivals," she said.
"I have a good relationship with them."
A night after successfully defending her 400 metres freestyle gold medal, Australia's Ariarne Titmus was all business as she returned to the pool for the 200m semi-finals.
She secured top seed ahead of second-quickest teammate Mollie Callaghan, the redheaded rocket who shares Titmus's coach Dean Boxall.
Titmus will bid to become the first swimmer to complete the 200m-400m "double-double" by going back-to-back in both events when she returns on Monday.
David Popovici was fastest into the men's 200m freestyle final with a time of 1:44.53, nearly half a second quicker than second seed Duncan Scott, Britain's most decorated Olympic swimmer.
South Africa's Tatjana Smith topped qualifying for the women's 100m breaststroke final, while U.S. swimmer Ryan Murphy will bid for another 100m backstroke title among five gold medals up for grabs on Monday.
