The home side had lost to Wasps at the Ricoh Arena in the opening game of their latest defence of the title, but they extended their unbeaten home record to 16 games in the tournament with a hard-fought victory.
Leinster arrived knowing that a third consecutive win in Pool One would end any hopes of reaching the last eight and they did not help themselves by having three forwards sent to the sin-bin. Steffon Armitage scored both tries for the home side from driving line-outs.
The home side, chasing a fourth successive title, paraded their latest big name signing, All Black World Cup winner Ma'a Nonu, for the first time in Europe.
The visitors were given the perfect start when former Saracens and England prop Matt Stevens collapsed the first scrum and Jonny Sexton hit the target from near the halfway line after only three minutes.
Early indiscretions by the champions allowed Leinster to build an advantage and Sexton doubled the lead with his second penalty in the sixth penalty after the home side had gone over the top at a ruck.
If the signs were encouraging for Leo Cullen's side, they got a reminder of Toulon's power 10 minute into the game.
Toulon's Steffon Armitage scored two tries in the defeat of Leinster.
Image credit: PA Sport
Having ignored a kickable penalty, and instead kicked into the right corner, they turned a line-out take by Samu Manoa into an unstoppable driving maul which resulted in Armitage crossing for a try which scrum half Eric Escande improved.
Escande then kicked a penalty to make it 10-6, but Seton replied to make it a one point game at the end of the first half.
Leinster survived 10 minutes without prop Cian Healy, who was sent to the sin-bin for a reckless challenge on Guilhem Guirado at a ruck - but it was a different story when Devin Toner followed him to the touchline six minutes into the second half.
This time Escande was able to add two more penalties to his tally and by the time the towering Ireland lock returned the home side had moved into a 16-9 lead.
There was one more penalty from the boot of the scrum half in the 67th minute which cruelly took away from the visitors any hopes of salvaging a losing bonus point.
Replacement lock Tom Denton becme the third Leinster forward to be shown a yellow card and no sooner had he gone off than Toulon scored another driving line-out try from Armitage.
There will be a 30,000+ crowd at the Aviva Stadium in Dublin when the two teams meet again next weekend - but with only one point from three games there is little or no chance of the Irish province going any further in the tournament.