Showing all their experience, Leinster timed their run perfectly, leading for just two of the 80 minutes as they eventually saw off a stubborn French challenge.
Having already won the title in 2009, 2011 and 2012, the Guinness PRO14 side returned to Europe’s summit in a game decided from the kicking tee, with Nacewa and Johnny Sexton combining to score all Leinster’s points.
Racing put pressure on Leinster early on, with Pat Lambie breaking through the middle but he injured himself in the process, going off after just three minutes.
Leinster had given away a penalty in that move, with Teddy Iribaren slotting the three points to give the French side the lead.
Leo Cullen’s side responded, and got themselves on the board with a penalty from Sexton after a clever backs move had put Nacewa away down the left and forced Racing offside.
Racing responded, earning their second penalty of the half when Leinster infringed at a maul, again Iribaren made no mistake.
Leinster could have levelled matters when they were awarded a penalty just after the half-hour. Instead Sexton chose to tap quickly and Luke McGrath was isolated, allowing Racing to clear their lines.
🔵 @leinsterrugby have done it!! The #GUINNESSPRO14 boys are the @ChampionsCup 🏆 winners as a late @racing92 drop-goal sails wide to the roar of the travelling Leinster fans. A phenomenal achievement 👏👏👏#ChampionsCup pic.twitter.com/Qu95zUAdNA
— PRO14 RUGBY (@PRO14Official) May 12, 2018
The three-time champions made up for it with three minutes left in the half though, Sexton punishing an intentional knock-on from Leone Nakarawa to make it 6-6.
As they had in the first half, Racing started the second quickly, and when Leinster strayed offside, Iribaren knocked over his third penalty.
Sexton looked set to do the same soon after but mishit his effort from nearly halfway. He made no such mistake shortly after to make it 9-9.
And on 55 minutes Leinster could have hit the lead for the first time following an infringement at a ruck which had all stemmed from a great lineout steal by James Ryan. Again however, Sexton could not convert from long distance.
Still, the momentum appeared to be with Leinster heading into the final quarter of the game as they started to monopolize possession and territory.
That edge was short-lived however, as Racing worked their way back into it. Iribaren missed his first kick of the game from halfway but rediscovered his radar with ten minutes to go after a collapsed maul to make it 12-9.
Leinster hit straight back with their most dangerous break of the half down the right, and when Racing strayed offside, Nacewa knocked over the resulting penalty to level it.
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.