Epaillard and his horse, French-bred stallion Usual Suspect D'auge, won a three-horse jump-off over eight fences in 40.67 seconds on the Champ de Mars, against the backdrop of the Eiffel Tower.
"It's amazing, here in Paris, in this special place, it is just fantastic," Epaillard, 39, said in a televised interview after seeing a high-risk ride being rewarded with victory on home soil.
"My horse is only nine years old, this is his first five star show," said the Frenchman, who learnt to ride in his parents' equestrian center.
"He was crazy good. It's a big surprise for me and I am so happy today."
Smolders regains overall lead
It was tough course in the sand arena, with the last two parts of a three-jump combination that was built close to the VIP stands falling seven times each. In the end, only three riders out of a field of 25 produced a clear round.
Belgium's Olivier Philippaerts, son of the country's most successful rider Ludo, and H&M Legend of Love finished second in 41.98 seconds while Harrie Smolders of the Netherlands aboard Don VHP Z took bronze with a calm and controlled ride in 42.13 seconds.
Television pictures after the jump-off showed Epaillard and Philippaerts had been even until the Belgian had a miscommunication with his horse between the second and third jumps which cost him the victory.
His podium finish meant Smolders regained the overall lead of the Longines Global Champions Tour (LGCT) from Christian Ahlmann. Smolders now has 185 points, five points clear of the German.
Italy's Alberto Zorzi, who claimed his maiden LGCT win in Monaco last weekend, lies in third place with 161 points.
The event, which attracted seven of the top 10 riders and the entire gold medal-winning French show jumping team from the Rio Olympics, was free to attend following a partnership between the LGCT and the Paris mayor's office.
Some 50,000 spectators were expected to attend what was the eighth leg of the 15-stop Longines Global Champions Tour, the world's richest for the top 30 riders in the world which will finish in Doha, Qatar in November.
Global Champions League
Earlier on Saturday, Belgium's Simon Delestre and his best friend Jérôme Guéry of France gave St. Tropez Pirates a win in the Global Champions League team competition.
It meant they took the overall lead from long-term frontrunners Valkenswaard United with 152 points and 133,931 euros in season prize money.
With seven more events to go, the team competition is going to the wire with the top three teams now separated by a mere three points.
The next leg of the LGCT will take place July 7 and 8 in Cascais, Estoril in Portugal.