Serving at 5 all, Nadal struck a down the line forehand winner followed by an overhead winner to lead 30-15. Then two backhand errors from Monfils put the Spaniard ahead 6-5.
Serving to stay in set one, Monfils struck an inside out crosscourt forehand winner but committed several baseline errors to trail 30-40.
A Monfils doublefault which hit the net then fell long would end the set 5-7.
A more aggressive Monfils broke serve at 5 all in set two to lead 6-5 after striking only one double-fault in the set.
Then serving at 40-15 with two set points, Monfils serve and volleyed.
Nadalís down the line backhand return landed wide to level the match at one set all after two hours and 21 minutes of play.
After a Nadal bathroom break, the Spaniard held serve the final set to lead 1-0.
And with the Frenchman looking more and more tired and winded, Nadal broke serve twice to race to a 4-0 lead.
Serving at 40-15, Nadal struck an inside out crosscourt forehand winner to lead 5-0.
Serving to stay in the match, Monfils struck two crosscourt groundstrokes long to trail 0-30 before hitting a crosscourt forehand winner to reach 15-30.
However Nadal then struck a down the line backhand volley winner to set up match point.
A Nadal down the line forehand winner ended the match and gave the Spaniard his ninth Monte Carlo title.
Between the years 2005 and 2014, Nadal won the French Open nine times to compile an astonishing 70-2 record on the clay in Paris.
In preparation for the French Open, this week Nadal is competing at the Mutua Madrid Open, a warm up ATP World Tour tournament at Madrid, Spain.