Leinster's 17-match unbeaten run in the Heineken Cup came to an end as Clermont Auvergne edged out the holders in a grinding encounter in France.
Morgan Parra kicked four penalties for Clermont with Brock James adding a drop-goal as the French side extended their run of home wins to a remarkable 51.
Parra and Sexton swapped penalties to leave the score at 9-9 but Clermont struck six points before the interval.
Despite a Sexton penalty, Clermont held on to take a five-point pool advantage.
Something had to give in Saturday's clash with Clermont undefeated at home in an amazing 50 matches and Leinster protecting a two-year unbeaten record in the Heineken Cup.
In fact, Leinster's last defeat in Europe's premier club competition had come against the French outfit in December 2010.
With Brian O'Driscoll, Rob Kearney and Luke Fitzgerald still among Leinster absentees, Clermont must have fancied their chances heading into the game - a repeat of last year's epic semi-final in the competition.
Clermont had the Leinster scrum under early pressure and Parra punished a Mike Ross indiscretion to put the French outfit ahead in the fourth minute.
However, Leinster responded in spirited fashion as an Ian Madigan break forced Clermont to concede a penalty in the seventh minute which Sexton slotted.
Sexton then missed a straightforward chance to put Leinster ahead and boosted by the let-off, Clermont regrouped to notch six quick points from Parra's boot.
However, Leinster were showing appetite for the fight and a rolling maul to the edge of the Clermont 22 set up a 29th-minute penalty which Sexton successfully kicked.
Sean O'Brien was producing an impressive return to action for Leinster while centre Andrew Goodman was also performing well on Heineken Cup debut for the Irish province.
A careless offside offence by Jamie Cudmore gifted three more points to Sexton in the 33rd minute but the parity didn't last long as Parra restored Clermont's lead four minutes later after a Leinster offence at the breakdown.
Leinster's good first-half endeavour was threatening to be undone right on the stroke of half-time as James drilled over a drop-goal to put six points between the teams again.
The holders produced pressure in the 10 minutes after half-time but it failed to produce any points.
Sexton's gamble of attempting to find the impressive Fergus McFadden with a quickly taken penalty from inside the Clermont 22 backfired in the 48th minute as the wing was unable to field the fly-half's diagonal kick.
However, Sexton did take his three points in the 53rd minute after a huge Leinster shove resulted in a penalty.
After a superb O'Brien tackle had halted Aurelien Rougerie in the Leinster 22, the Irish province wasted a chance at the other end as replacement hooker Richardt Strauss produced a crooked line-out throw as the holders threatened.
Both teams produced moments of threat in the closing stages and Leinster had to defend bravely in the final play of the game to hold on to the losing bonus point ahead of next Saturday's return clash between the sides.
Clermont Auvergne coach Vern Cotter described the contest as a "good old boxing battle".
"We exchanged blows from the start to the end and luckily sneaked away with a three-point advantage.
"We're happy with the win but not probably with the way we played in the second half.
"Hopefully our confidence levels will increase and we'll be able to go to the Aviva and not get treated the same way we did two years ago."
Leinster coach Joe Schmidt described his team's defeat as "incredibly disappointing".
"We were close enough to steal it. We dominated territory in the second half and created some really good opportunities but we just were not precise enough.
"We lost a couple of key balls on the line-out throw and that didn't allow us to keep the pressure on them.
"But I was happy with a lot of what we did."
Clermont Auvergne: L Byrne; S Sivivatu, A Rougerie (cpt), W Fofana, N Nalaga; B James, M Parra; V Debaty, B Kayser, D Zirakashvili, J Cudmore, N Hines, J Bonnaire, A Lapandry, D Chouly.
Replacements: R Chaume for Debaty 53, T Paulo for Kayser 59, D Kotze for Zirakashvili 60, J Pierre for Cudmore 66, J Bardy for Bonnaire 78
Not Used: L Radoslavjevic, D Skrela, R King.
Leinster: I Madigan, F McFadden, G D'Arcy, A Goodman, I Nacewa, J Sexton, I Boss; C Healy, S Cronin, M Ross, L Cullen (cpt), D Browne, K McLaughlin, S O'Brien, J Heaslip.
Replacements: E Reddan for Boss 65, R Strauss for Cronin 48, D Toner for Browne 56, S Jennings for O'Brien 65
Not Used: J McGrath, M Bent, F Carr, A Conway.
Referee: Nigel Owens (Wales)