The 22-year-old is one of France's big hopes for gold as he takes on the twin challenge of the 200 metres and 400m medley -- and potentially both 200m butterfly and breaststroke.
There will be no shortage of support for a young man who used to cycle the few hundred metres between his family's home in Toulouse in southwest France to the local swimming pool.
It was hardly a surprise to see the little blonde six-year-old pedalling to the Art Deco building that housed TOEC (Toulouse Olympic Employees Club): he was effectively born to water.
His father, Xavier Marchand, swam medley at the 1996 and 2000 Olympics and won a silver behind Marcel Wouda at the 1998 world championships.
His mother, Celine Bonnet, was also a top-level swimmer, who took part in the Barcelona Olympics in 1992, at the same time as Leon's uncle, Christophe Marchand.
In spite of the genes and early access to the local pool, Leon took a while to adapt to water: he briefly abandoned the pool to try his hand at judo and the region's most popular sport, rugby.
'Natural leader'
When he returned to swimming, it was of his own volition, his parents leaving him to find his own path. If anything they tempered his desire to swim, aware more than anyone of the demands of the sport.
Nicolas Castel began training him when Marchand was just eight and remembers a "quiet little boy" who was "not very advanced physically".
"In terms of motor skills in the water, he was pretty good, but not exceptional either," said Castel.
"Above all, he was happy to be with his mates.
"That's been the common thread running through his career so far. Wherever he's been, he's been able to create a group of friends with whom he lives an adventure and shares good times."
Remi Lacourt is one of them. A friend of Marchand from secondary school and the swimming club, he too remembers a physically "underdeveloped" boy.
"He must have been 40kg in sixth grade (11 years old), he was a shrimp," said Lacourt, who added that Marchand was always smart.
"He became aware very early on of the importance of streamlining," he said, referring to the period a swimmer spends underwater after the initial dive and each turn which remains one of Marchand's great strengths.
"And he was a natural leader. He wasn't necessarily the one who spoke the loudest or had the strongest opinion.
"He was good at everything he did. We agreed in ninth grade (14 years old) that he would be elected class delegate, but he didn't run."
'Strong, protected'
Most of the swimmers in his group gave up swimming as the years went by but Leon ploughed on, length by length, progressing through the local and national youth set-ups all the way to the Tokyo Olympics, where he came sixth in the 400m medley, and on to Arizona State University.
"There wasn't a click, a moment when we said to ourselves: 'This is it, this is the future Michael Phelps'," says Castel.
"It happened gradually. Every time we set ourselves a goal, we achieved it.
With earrings in both ears and tattooed arms, the young coach, himself a former swimmer with the TOEC Dolphins, has grown up alongside his protege, without Leon's parents interfering in his work.
"Xavier and Celine understood very quickly that it wasn't necessary for several sources of advice to cross paths," he stressed.
"That's also what made Leon so strong. He felt good, protected."
When contacted by AFP, former champion Xavier Marchand, now a videojournalist with France 3 Occitanie, preferred to keep his distance from his son: "It's his story, not mine".
The story of Leon Marchand has only just begun: he will be looking to add a new chapter at the La Defense Arena at the end of July.
