Del Potro, beaten in last year's final by Roger Federer, lost his serve in the first set but Benneteau had no answer to his solid baseline performance which earned him the tiebreak 7-2.
After an early break in the second set Del Potro never looked back against 39th-ranked Benneteau, still seeking a first title having lost all eight finals he has contested.
"It was so important to win the tie-break as I missed several opportunities through mistakes I made to clinch the first set sooner," Del Potro told reporters.
Del Potro became only the second Argentine after Guillermo Vilas to triumph at the tournament.
"I had a great week and really enjoyed the tennis I played from the first round on," added Del Potro.
Benneteau said he had fought with all his might to match the hitting and power of world number seven Del Potro.
"It was tough to stay in the first set as I was under pressure every game, as well on my own serve as his," said the Frenchman, who beat Federer in the last eight.