Marchand was little known outside the swimming world before these Olympics, but he has become a national and international phenomenon in the space of a week with a quartet of titles.
With the 400m medley, 200m butterfly and 200m breaststroke already in the bag, the 22-year-old rose to the occasion again in front of French president Emmanuel Macron in the 200m medley at a deafening La Defense Arena.
Roared on a partisan home crowd, he hit the wall first in 1mins 54.06 -- the second fastest time in history -- to ensure he will leave the Games as one of its biggest stars.
"It's only the beginning. I'm really excited... my next goal is LA," he said, referring to the 2028 Olympics in Los Angeles.
"Last night I slept like 11 hours. I was just so tired.
"But this morning I woke up, I was full of energy again. I don't think anything went wrong this week. It was just perfect."
McKeown, 23, did not generate quite the same reception but her feat was also exceptional.
She held off American arch-rival Regan Smith to win the 200m backstroke in an Olympic record 2:03.73, on the back of her 100m victory, to remain flawless over two Olympics with four golds from four individual events.
No Australian swimmer, man or woman, has ever completed a "double-double" -– defending two different individual Olympic titles.
Her teammate, Cameron McEvoy, thundered to the men's 50m freestyle gold in 21.25sec, with defending champion Dressel only managing sixth. Britain's Ben Proud took the silver.
Dressel, who won five golds at the Tokyo Games in 2021, returned to the pool but failed to qualify for his 100m butterfly title defence.
Five-time world champion Marchand went into his race as favourite and with fans ready to accept nothing less than victory, he emphatically delivered, just missing Ryan Lochte's 13-year-old world mark.
China's Wang Shun powered ahead after the opening butterfly leg, but Marchand then switched through the gears to assume control, with Macron on his feet pumping his fist in celebration at the finish.
Defending champion Wang came third, with Tokyo silver medallist Duncan Scott of Britain second again.
Cheptegei holds strong
Uganda's Joshua Cheptegei withstood Ethiopian team tactics to claim gold in the men's Olympic 10,000m at the Stade de France.
The three-time world champion and world record holder, silver medallist at the Tokyo Games, timed an Olympic record of 26min 43.14sec for victory.
That beat the previous best of 27:07.17 set by Ethiopian great Kenenisa Bekele at the 2008 Beijing Games.
Ethiopia's Berihu Aregawi edged fast-finishing American Grant Fisher by two-hundredths of a second to take silver in 26:43.44.
Cheptegei admitted to having been inspired by Bekele's Beijing triumph.
"It's the dream of young people to achieve what they want to achieve in life," said Cheptegei.
"Barely 16 years ago when I was watching the great Kenenisa Bekele win in Beijing, it was something that grew in my heart.
"I said, one day, one time, I want to be Olympic champion."
Page bags full set
Outside of the pool, Britain's Bryony Page claimed gold in the women's trampoline in a nerve-wracking final at the Bercy Arena on her third Olympic medal-winning appearance.
She was fifth in qualifying and had long been on a quest for gold, having won the silver at the 2016 Rio Games and bronze at the Tokyo Olympics.
Page, the seventh of eight competitors to jump in the final, said she was extremely nervous when awaiting her score, which amounted to 56.480 points.
She was so thrilled to see herself in first place that she thought she had already won gold.
"I forgot there was another competitor to come," she said.
British showjumping gold
British showjumpers Ben Maher, Harry Charles, and Scott Brash won Olympic team gold, ahead of the USA and France.
Securing their third team win in the discipline with just two time penalties and zero knocked-off rails, Britain was ahead of the United States with four and France with seven penalty points.
The victory added to Britain's team eventing gold earlier in the week.
Charles, on 15-year-old bay gelding Romeo 88, had one of the day's best performances, incurring no penalties.
