Sinner's first ATP Tour final had come at this event in 2020, where he opened his account by beating Vasek Pospisil in three sets, a sterner test than he was given by Monfils.
The Italian had since tasted victory twice more, while Monfils' previous final had been in February 2020, although his success this week extended a run of seasons with final appearances to 17 in a row.
The most recent three of the Frenchman's prior 31 had ended in glory, but he was no match for defending champion Sinner.
The 20-year-old had already passed up a break point before consecutive superb forehand winners brought the breakthrough, the second clipping the net on its way through.
A marathon rally saved the second of two break points Sinner faced in the seventh game, yet that was as close as he came to being stopped.
After Sinner served out the opener, Monfils paid the price for losing his focus at the start of the second, adamant he had landed the first point of the set inside the line as a narrow call went against him.
Monfils's wild backhand then went long to give Sinner the lead in the second, one he never relinquished as he coasted through to serve out the match to love.
"I'm happy to be one more time the winner here in Sofia," Sinner said. "I think it's a very, very nice tournament.
"For sure it was the best match [he played in the tournament], because I had to. It's never easy to play against him. We've had very tough matches, I think everyone knows that.
"Today, in the [key] moments, I was a little bit luckier than him, so that helped a lot."