French giants Toulouse had averaged 6.6 tries and over 44 points per game in the competition this season on their way to Saturday's showpiece match at London's Tottenham Hotspur Stadium.
But it was defence that was the foundation of their record-extending sixth Champions Cup final triumph.
Toulouse made 242 tackles to their opponents' 137 across two gruelling hours of rugby, with Leinster completing an unwanted hat-trick of defeats after losing both the 2022 and 2023 Champions Cup finals to another French club, La Rochelle.
Neither Toulouse nor Leinster managed a try in a regulation 80 minutes that ended level at 15-15, following five penalties apiece.
Toulouse wing Matthis Lebel scored the first try of the match early in extra time before team-mate Richie Arnold was sent off for a dangerous clear-out at a ruck.
Leinster hit back with a try from Ireland back-row Josh van der Flier, but Toulouse replacement Thomas Ramos' two penalties in the second half of extra time proved decisive.
"The feeling is a great joy," player-of-the-match Dupont told reporters. "There are feelings and emotions that are hard to put into words."
The 27-year-old France star, already regarded as an all-time great scrum-half, added: "We've worked so hard... Extra time is crunch time, money time, and you shouldn't ask questions, just get your head down and do what you need to do.
"You win the game mentally."
For his half-back partner Romain Ntamack, there was the satisfaction of equalling his father's record of two Champions Cup titles as a Toulouse player, with Emile Ntamack involved when the team won the inaugural 1996 final.
"As a kid, I always wanted to achieve what he did and now it's nice to be writing another chapter of Toulouse's history," said Ntamack junior.
Toulouse now have a chance at completing a European and domestic double this season by retaining France's Top 14 title.
But coach Ugo Mola, who praised his side for making "precious turnovers close to our line" against four-time European champions Leinster, said talk of the double was "presumptuous".
"We need to make the semi-finals before we talk about the double," he added.
'Devastated'
Leinster boss Leo Cullen, meanwhile challenged the Dublin-based side to overcome yet more European heartache by winning the domestic United Rugby Championship.
"Everyone is devastated. We've been on the losing side on a few occasions now," Cullen said.
"We need to see what we're all made of now, as hard as that will be. We'll process it and move on to the next challenge against Connacht on Friday night. They won't make life easy for us."
He added: "There's not a lack of belief that we can do this, it's just making sure that we stick at it because we will get across the line."
Leinster replacement Ciaran Frawley kicked the penalty that secured extra time, but he was also just off target with a late drop-goal that would have snatched victory.
"It comes down to the finest thing," said Cullen. "There was a drop-goal at the end and if that goes a metre to the right then we're having a very different conversation here. That's the nature of sport at the top end.
"Toulouse scrambled well and came up with big moments defensively and that was the difference... We just felt we were so close to breaking them and most people in the stadium would have that sense."
