Mauresmo, the third seed, defeated the Russian fourth seed 7-5, 2-6, 7-5 as both players warmed up for the upcoming WTA Tour Championships in Los Angeles.
After a poor second set, Mauresmo held a 4-1 lead in the third before Dementieva reeled off four straight games to serve for the match. But the Frenchwoman - twice two points from losing - broke back, held and broke again to win the match.
"There were a lot of ups and downs for me but also for her," Mauresmo said. "I lost my rhythm in the second set and felt like I let her back in the match. I wasn't aggressive and started to feel it physically, but I still managed to stay in control of the points in the third."
Mauresmo will be taking some momentum into the season-ending event. On Saturday, she eliminated Czech Nicole Vaidisova, who had won 18 straight matches.
It was the third title of the season and 18th career for Mauresmo, the fourth-ranked player in the world.