The Irish province now jointly hold the record for most European Cups won along with Toulouse, while Cullen is the first man to win the tournament as both a player and a coach. Racing had started strongly and the South African's linebreak had Leinster scrambling, with Johnny Sexton making the tackle on the Springbok, who seemed to turn his ankle as he went ground.
The French side took the lead four times throughout the contest at the San Mames Stadium in Bilbao, thanks to their impressive scrum-half Teddy Iribaren.
We couldn't get going in the game.
And he did it again with two minutes remaining when Racing infringed in front of the posts, with Rémi Talès unable to take the game into extra-time as he pulled a late drop goal attempt wide.
It completes a dream season for the former Cooley Kickhams GAA player who was also part of Ireland's Six Nations Grand Slam in the Spring.
The second half played out to a similar pattern, with Iribaren and Sexton exchanging penalties at the atmosphere grew increasingly tense in northern Spain.
That lead lasted just over 10 minutes as Nacewa launched a Leinster attack down the left wing that led to a penalty for the Irish province. The 27-year-old former Brive scrum-half took the kick, showing ideal composure to give his side the first three points of the game. Iribaren made it 6-3.
Sexton badly fluffed a long-range effort as Leinster, for all their territorial dominance, couldn't quite find their rhythm amid Racing's desperate, hard-hitting defence.
Sexton levelled the scores with a penalty of his own in the opening quarter and the two kickers would exchange further three-pointers to leave the scores level at 6-6 at the break.
With seven minutes on the clock, Wayne Barnes penalised Racing and it was Nacewa that stepped up, instead of Sexton, to level the game at 12-12.
Leinster got the ball back - Racing were happy to let them make most of the running - and Sexton redeemed himself on 53 minutes with a penalty to make it 9-9.
In between times an absorbing and brutal, if at times ugly, first half had played out in Bilbao as Leinster counteracted Iribaren's sniping with a kicking game that targeted their opponents' back three's lack of height.