A finalist in the French Open juniors and doubles champion in Roland Garros in 2021, the youngster from outside Paris broke through having reached the semi-finals in Marseille and Montpellier this season.
"I will remember this forever," said Fils who took almost three hours to win his semi-final in three sets the day before.
"I'm happy with the win and now I want more. It was a tough semi-final match. Today I came onto court with not lots of energy, but I was focused on my game and trying my best."
Fils sealed victory on his first match point against the fourth-seeded Argentine, collapsing in disbelief as the home crowd cheered.
The Frenchman also broke into the top 100 for the first time, rising 49 places to 63.
Fils and countryman Luca Van Assche, 19, are the only teenagers in the men's draw at Roland Garros next week.
Cerundolo was aiming to capture his second tour-level trophy after Bastad last season.
He will arrive at the French Open with a 13-9 record on clay in 2023, after reaching quarter-finals in Rome and Barcelona.