The world number three, riding Cortes C, came out on top after an impressive jump-off, beating Christian Ahlmann and Scott Brash to victory.
Still, second place for Ahlmann was enough to propel the German world number one to top the provisional GCT standings.
With 18 of the world's top 20 in attendance at the event in the magnificent surrounds of the Renaissance chateau just north of Paris, a tough competition was expected.
But it was the Uliano Vezzani-designed course that proved to be the biggest challenge to riders and horses, and there were several upsets in the opening round, most notably Laura Kraut - championship leader heading into Chantilly - who had 12 faults.
In the jump-off, Swiss Jane Richard Phillips set the early tone with a risky u-turn in the middle of the course to link two fences and a long gallop to reach the last one on her way to a clear round.
The young rider, who continues to show signs of improvement, stopped the clock at 44.86 seconds, and ended up in fifth place.
Meanwhile, two French riders, Roger-Yves Bost and Jérôme Hurel, both failed to clear, despite the support of a vociferous home crowd. Hurel even had a brief scare when he fell off Ohm de Ponthual in the double combination.
In end though, the day belonged to Madden, whose performance drew 'oles' from the crowd.
"The course was more difficult than it seemed with nine double clears," Madden said. "It was a fantastic jump off and I had to go out. My horse was fantastic. He makes big jumps seem little.
"I love a big field like this, great atmosphere with the public and top riders. I turned it on from 1-2 and my horse was fantastic to run to the last like that. I am in a lucky spot in my career. I have some fantastic horses. It will be a goal to be number one (in the rankings)."