In a fittingly seismic clash between the tournament's most successful teams, Saturday's game was locked at 15-15 after 80 minutes before winger Matthis Label broke it open with the first try of the match.
The game swung back Leinster's way when Australian 2023 World Cup lock Arnold was red-carded for a dangerous hit in the 89th minute.
Richie Arnold collides with Leinster's Cian Healy before being shown a red card for the challenge. (AP PHOTO)
Leinster took immediate advantage with a pushover try by Josh Van der Flier that needed a lengthy TMO intervention. Ciaran Frawley converted to reduce the deficit to 25-22 at halftime of the extra period.
Toulouse then claimed a decisive stranglehold with two Thomas Ramos penalties to take the wind out of Leinster's sails and hand the Irish province a third successive final defeat.
"It is awesome to be able to win with this team," said Toulouse's man of the match Antoine Dupont. "We really wanted to win the sixth star after losing to Leinster over the last two years (in the semi-finals)."
Toulouse skipper Antoine Dupont holds up the Champions Cup trophy as teammates celebrate. (AP PHOTO)
The match between two of the best attacking teams in the world, both appearing in the final for the eighth time, proved to be more of an arm wrestle as Toulouse led with two penalties from Blair Kinghorn.
Leinster got a foothold but Toulouse's ferocious defence - typified by a superb try-saving tackle by Kinghorn on Dan Sheehan - and aggressive work at the breakdown restricted them to a Ross Byrne penalty in the first 38 minutes
Toulouse had averaged 6.6 tries and over 44 points per game in the competition this season but were happy to opt for another Kinghorn shot from the tee, before Byrne replied to make it 9-6 at halftime.
Two more penalties made it 12-12 before Toulouse thought they had scored a spectacular try when Lebel caught Romain Ntamack's crossfield kick but Jordan Larmour had done just enough to force his foot into touch and the TMO ruled it out.
The French side were ahead soon, however, through a penalty from replacement Ramos, but with three minutes remaining Leinster earned a scrum penalty that replacement flyhalf Frawley slotted to make it 15-15.
His attempted drop goal to win it floated wide and the final went to extra time for the third time.
Inspired by the increasingly influential Dupont, Toulouse continued to drive forward and Leinster cracked when Santiago Chocobares burst clear and allowed Lebel to turn on the after-burners and score the opening try before the Arnold red card added to some late jitters.
"We're pretty devastated. We poured a huge amount into this game," Leinster head coach Leo Cullen said.
"It was a game that was about taking opportunities, we had plenty of opportunities, some we couldn't quite nail. It's important we show character as a group now. Toulouse showed the champion qualities they've got."
