But Sexton responded with a penalty and Iribaren's fourth penalty was cancelled out by Nacewa's boot.
The second half got off to a poor start for Leinster as Isa Nacewa squandered possession, Racing thundered forward and Iribaren knocked over another simple penalty when Barnes pinged the Blues at the breakdown.
As they had in the first half, Racing started the second quickly, and when Leinster strayed offside, Iribaren knocked over his third penalty.
Lancaster, who works under Cullen as senior coach, joined the Irish province in 2016, having resigned as England boss in the wake of their disappointing World Cup campaign in 2015, and Sexton said he has made a big impact.
Nacewa, Cian Healy, Sexton, and Devin Toner all collected a record-equaling fourth title.
A pre-match hamstring injury spoiled Carter's European swansong, with Racing's injury list already including talismanic skipper Maxime Machenaud (knee) and Dimitri Szarzewski (bicep).
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.
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He's just unstoppable. "But we just gotta be mentally locked in and have the mindset to come in and try to do it the best we can". You look at the Patriots. "It's a sports town, you look at the Patriots, you look at the Bruins, you look at the Red Sox".
The Irish province's veteran New Zealand-born captain Nacewa, who retires at the end of the season, broke the line, but the winger capped once by Fiji was hauled back by opposite number Teddy Thomas.
Although struggling to produce quick ruck ball on the greasy surface, Leinster had the edge under the high ball through Rob Kearney.
Racing took what appeared to be a decisive lead with less than 10 minutes remaining when Iribaren slotted a nerveless three points. Sexton slipped in his attempts to respond with a hard 48-metre penalty.
Leinster had been deadlocked with Racing for the whole game, and looked as though they may have been left to rue two missed kicks by Johnny Sexton.
The Ireland fly-half nailed his next effort to square things up again in the 53rd minute. Again however, Sexton could not convert from long distance.
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.