Sinner, 20, added momentum to his chances at qualifying for the Nitto ATP Finals as he won his third career title, moving up to become world No. 10.
The Italian broke Monfils' serve early in the first to race to a 3-0 lead before maintaining his composure to claim the opening set. He continued to dominate in the second and retained a flawless record by not dropping a set this week as he eased to victory.
Sinner paid tribute to Monfils in his winner's speech, adding that he is looking to finish the season strongly.
"First of all, Gael it was a great week," Sinner said. "Unlucky in some moments today but we had some great fights, great matches.
"Obviously congratulations to you and your team. Tennis needs you, it really needs you so thank you for staying here in tough moments without crowds. I know you don't like that but soon everything, hopefully, is going to be normal so good luck to you."
Information from the Associated Press contributed to this report.