Sexton’s Racing Metro appeared down and out as the Scarlets led 23-10 in the second-half following tries from Scott Williams and Rhodri Williams.
But the nouveaux riches club fought back thanks to the boot of the Ireland ace and a Masi Matadigo try.
Sexton was uncharacteristically off target with a potentially match-winning penalty with just seven minutes left on the clock.
And minutes later the TMO adjudged he had knocked on in the process of stripping the ball out of contact in the build-up to Wenceslas Lauret’s disallowed second-try.
According to the rule-book the ball can go forward in the process of ripping the ball from contact but Iain Rammage felt Sexton fumbled the ball on the way to the ground.
Sexton said: “It’s bitterly disappointing not to get the win. We were down to 13 men and losing by 13 points but managed to claw our way back and did well never to give up.”
The Scarlets remained top of Pool Four on points difference with Racing Metro in second.
Sexton insists all is to play for in the race for qualification.
He added: “It’s all to play for us with one home win and a draw away. All the teams can beat each other, which is the good thing about this pool.”
SCARLETS: L Williams; N Reynolds, J Davies (capt), S Williams, J Williams; R Priestland, R Williams; P John (R Evans 69), E Phillips, J Adriaanse, J Ball (J Snyman 64), G Earle, A Shingler, J Barclay, J Turnbull.
RACING METRO 92: B Lapeyre (B Dambielle 69); A Plante, H Chavancy, A Dumoulin (F Estebenez 22), J Imhoff; J Sexton, L Magnaval (R Snyman 57); S Tonga’uiha (E Ben Arous 60), D Szarzewski (capt), L Ducalcon (J Brugnaut 66-69), J Kruger, F van der Merwe (F Metz 61), W Lauret, D Lydiate, M Matadigo (V Lacombe 41-43, C Gerondeau 57).
Referee: Neil Paterson (Scotland)