Aubry said he needed a 'miracle' to qualify after finishing his heat and such was the level of the competition that Tokyo bronze medallist Mykhailo Romanchuk of Ukraine and 2023 worlds runner-up Samuel Short of Australia failed to make the cut.
The Frenchman had finished 29th fastest out of the 33 swimmers in the event at the Tokyo Olympics but he redeemed himself in front of a roaring home crowd by winning his heat and going eighth-fastest overall.
"You saw the experience I had at the Tokyo Games, it was really horrible for me, I erased that completely from my mind because there I missed things completely," Aubry told reporters.
"So here, at the home Paris Games, it's something incredible. In Tokyo we didn't have any fans in the audience which means that swimming like that all alone was very, very hard knowing (how the race was going).
"And I was sick, I wasn't in great shape. I restarted again two years ago, I set myself the Paris Games as a goal. I'm here today, so it's a real joy."
Ireland's Daniel Wiffen topped the time charts with seven minutes and 41.53 seconds but Aubry said he was happy with his 7:44.59, especially after going faster near the end of the race.
"I have no regrets, that's my second-best time. I felt really good, I've got a really good feeling about the final," he added.
"I discussed it with my coach and we said that I have to do the first 400 (metres) really comfortably, technically well, which I did. In the second 400 I picked it up little by little and it paid off."
The final will be held on Tuesday.
