Harlequins edged out Montferrand 27-26 to claim the Parker Pen Challenge Cup title as well as a place in next season's Heineken Cup.
PARKER PEN CHALLENGE CUP
Try: Azam, Mignoni
Con: Froch (2)
Pen: Froch (3)
Try: Duffy, Keogh
Pen: Burke (4), Dunne
A last-gasp try from Simon Keogh left Quins one point behind and Andy Dunne slotted the crucial extras.
Olivier Azam and Pierre Mignoni touched over for the French, who had Raphael Chanal sent-off for punching.
Prop Jason Leonard came off the bench to win his 219th appearance for Quins in his last competitive match.
In a tough opening half, Paul Burke and Anthony Floch swapped penalties, before Quins took the lead.
Burke's lofted kick caught out Sebastien Kuzbik and a generous bounce allowed Gavin Duffy to stride over for the opening score.
The French side looked agitated on the pitch and were guilty of a number of handling errors.
In the set piece, however, they proved devastating, driving Quins back in the opening scrum before winning the second against the head, allowing Azam to drive over.
Floch added the extras and, seconds later, fly-half Gerald Merceron pushed his side further ahead with a long-range drop goal.
With half-time approaching Burke then kicked his side into a slender 14-13 lead with two well-timed penalties.
But Montferrand had the last word when Froch slotted his second three-pointer as the half-time whistle was blown.
Quins started the second half as they had the first, with George Harder providing power out wide and Burke bossing the midfield well.
The fly-half once again gave his side the lead with his fourth penalty of the afternoon.
Harlequins will comepete in next year's premier European competition
Sébastien Kuzbik came close to making amends for his first half error when he strode over on the left, only to be brought back for a forward pass by referee Nigel Whitehouse.
A shaky Mel Deane was then forced off the field after an attempted tackle on Aurélien Rougerie.
With Montferrand proving too strong up front, it was only a matter of time before they crossed again and when the pack rumbled forward from a close line-out, scrum-half Mignoni touched over with ease.
Floch slotted the extras but again Montferrand let themselves down when Chanal was given a yellow card for an attempted punch.
Burke hobbled off to be replaced by Dunne, who instantly pushed his side back within three points, only for Floch to cancel it out with his third penalty of the day.
A delicate chip and chase from Will Greenwood then released Ugo Monye, but Rougerie proved up to the task defensively to the dismay of the home fans.
In his 219th appearance - and his last - for Quins, Jason Leonard then replaced Jon Dawson as Chanal was given his marching orders with another yellow card.
With Montferrand reduced to 14 men, Quins put together their best move of the match to allow replacement Keogh to cross underneath the posts.
And when Dunne's kick flew true, it prompted a night of celebrations for the impressive Quins support.
Chanal later received a two-week suspension for his sending off.
Harlequins: Duffy; Harder, Greenwood, Deane, Monye; Burke, Bemand; Worsley, Fuga, Dawson, Dawson, Miall, Evans, Sanderson, Vos (capt), Diprose.
Replacements: Hayter, Jones, Leonard, Davison, Sherriff, Keogh, Willis, Dunne.
Montferrand: Floch; Rougerie, Chanal, Marsh (capt), Kuzbik; Merceron, Mignoni; Soulette, Azam, Attoub, Louw, Privat, Raynaud, Magne, Vermeulen.
Replacements: Cockerill, Emmanuelli, Pearce, Audebert, Dieude, Moreau, Viars.