It has taken Nadal four years to wait for crowning in the Principality and almost two years for an ATP 1000 Masters title, Xinhua news agency reported.
Nadal, who won in Monaco for eight straight time from 2005-2012, beat the 13th seed Monfils 7-5, 5-7, 6-0 in his 100th career final.
The victory marked the No.5 seed’s regurn to form after struggling for fitness since losing the 2013 final to Novak Djokovic, and he equaled the Serb’s record of 28 Masters titles with the trophy.
“It was a very important week for me. I feel much better than I did last year. The victory here confirms that I’ve been better,” said Nadal.
“Monte Carlo is one of the most important places in my career and it’s so special to win here after three years.
“Winning nine times here is unbelievable, and competing well two weeks in a row is something positive,” he added.
The 29-year-old exchanged consecutive break points in the first set which Nadal won 7-5, before Monfils broke Nadal to take an early 2-1 lead in the second and took it 7-5, though the Spaniard broke back to level at 4-4.
Nadal then broke three times to dominate the final set 6-0 and wrapped up his 48th final victory on clay.
Monfils, the first Frenchman having reached the Monte Carlo final since Cedric Pioline crowned at the tournament in 2000, had been looking to win his first Masters title after runner-up finishes in Paris in 2009 and 2010.
“I knew Nadal was confident. He’s a great champion and puts more into it at the right moments. You’re just submissive to his power. Suddenly in the third set, he increased his power and I just couldn’t cope,” he said.