Racing, who lost former New Zealand international Dan Carter to injury prior to the game, gave their all throughout the contest and many were surprised they managed to hang in for so long in the contest.
The two teams couldn't be separated at half-time, with Irish star Johnny Sexton's three penalties for Leinster matched by French scrum-half Teddy Iribaren, who was handed kicking duties for the game.
Both sides toiled after the break and Racing, through Iriabern's fourth kick, went 12-9 ahead with nine minutes to play before Nacewa slotted over two late penalties to give Leo Cullen's men a 15-12 victory.
Leinster have now lifted the trophy four times, becoming the joint most successful team alongside Toulouse, and their New Zealand match-winner expressed his delight after the match.
Speaking to BT Sport, Nacewa said: "Racing are world class. Johnny Sexton said that it would be a Test match all week. They have got the best defence in the tournament, but fortunately we have got the genius Johnny to marshall us around the pitch."