It was Racing who nudged ahead once again heading into the final 10 minutes thanks to Iribaren, but this time it was Nacewa - retiring at the end of the season - who provided the prompt riposte before kicking the victor at the death, Tales unable to find the target with his hopeful drop-goal attempt.
Leinster maintained their flawless run in European Champions Cup finals with a last-gasp 15-12 win over Racing 92 in Bilbao to make it a record-equalling four titles.
Captain Isa Nacewa and Sexton kicked Leinster to an absorbing 15-12 victory, with Nacewa's final kick, less than two minutes from the end, putting the victors ahead for the first time in a contest that failed to get going in an attacking sense. Leinster in front for the first time in the game.
Carter was ruled out of this morning's (NZT) final due to a hamstring injury. The Teddy Thomas-inspired Racing 92 stand in their way.
A high Garry Ringrose tackle following Lambie's early injury saw Teddy Iribaren, deputising for the injured Maxime Machenaud at scrum-half, open the scoring with a penalty in a tight game where defence was the victor and wide, expansive attack never an option.
Racing opened the scoring as Sexton and Iribaren traded penalties with the Irish province finding it hard to settle into the game in the opening 20 minutes.
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Penalty goals also coming from Sexton in the first and second halves.
Racing took what appeared to be a decisive lead with less than 10 minutes remaining when Iribaren slotted a nerveless three points.
From a Leinster line-out, Racing's Leone Nakawara stole possession superbly.
Leinster: 15R Kearney; 14 J Larmour, 13 G Ringrose, 12 R Henshaw, 11 I Nacewa (capt); 10 J Sexton, 9 L McGrath; 1 C Healy, 2 S Cronin, 3 T Furlong; 4 D Toner, 5 J Ryan; 6 S Fardy, 7 D Leavy, 8 J Murphy.
Racing came close to forcing extra time after the buzzer when stand-in playmaker Remi Tales's drop goal flew just wide.