Mannarino had a tough start in the tournament when he saved four match points in his opening match but bounced back in fashion by beating four seeded players en route to his first tour-level final since 2020.
The 34-year-old then finished the job by beating Laslo Djere 7-6 (7-1) 6-4 in the final, picking up his second tour-level title.
"That was not expected after the first match because I had such a long one, and I knew if I wanted to go through the entire tournament, I would have to play every day for another five matches," Mannarino told reporters.
"I just came here with the ambition to play some matches before the US Open, and I am really happy that I was actually able to do more than that by winning the tournament. It's unexpected but it's good news."
Mannarino became the oldest winner in the tournament's history and also its first French champion.
"This is not such an important thing to me. I am just happy I won the tournament - no matter who won before or who's going to win after," Mannarino said.
"It's just something that I've (accomplished) for now and I'm really happy with that."
Mannarino is due to face the Netherlands' Gijs Brouwer in the first round of the US Open, the year's final grand slam, which begins in New York on Monday.
