The 2004 Olympic 400 metres freestyle champion, who quit swimming in 2009 after becoming pregnant, returned to the sport last year in an attempt to take part in this year's Games.
The 25-year-old clocked 1:00.18, well below the required 1:00.82 Olympic A time, almost four years after failing to shine at the Beijing Olympics.
The top two in each final at the French championships qualify for the London Games providing they swim under the Olympic A time.
"I would like to thank my family, my friends, and all those who have always believed in me," she told Eurosport.
"It was not an easy challenge, but the French crowd helped me a lot. In 2004 (in Athens), it was easy, I was 17, I was crazy."
France showed they would be a force to be reckoned with in London when world season's bests were set in the 200 freestyle.
Yannick Agnel set a French record and world season's best of 1:44.42 to book his place in London along with second-placed Amaury Leveaux who finished 0.30 off the pace.
The day after swimming the season's fastest 400 metres freestyle, Camille Muffat again impressed by winning her 200 freestyle semi-final in a French record time of 1:55.40.
"I'm very happy, I did not think I could swim that fast today especially since I had trouble sleeping last night," she told reporters.
World champion Jeremy Stravius, however, failed to qualify when he finished third in the 100 metres backstroke final behind co-world champion Camille Lacourt and Benjamin Stasiulis.