Fit-again captain Jonathan Sexton was back in command as Leo Cullen's men set up an intriguing decider with unbeaten Saracens at St James' Park on Saturday May 11.
Thomas Ramos' penalty brace kept Toulouse in the contest, with both sides coping well during first-half sin-binnings for Richie Gray and Robbie Henshaw.
Leinster also absorbed the losses of an ill Rhys Ruddock, who was withdrawn pre-match, and the tournament's joint-top try scorer, Sean Cronin, who injured his right leg in the 33rd minute.
Ruddock's replacement Scott Fardy crossed in the 52nd minute, sandwiched by penalties from Ramos and replacement Romain Ntamack.
Man-of-the-match Sexton completed his 12-point haul with 15 minutes remaining, Leinster keeping a frustrated Toulouse try-less and adding a late Ross Byrne penalty as they advanced to their fifth European final in 11 years.
Young full-back Ramos curled over a fifth-minute penalty, rewarding a powerful start from the resurgent French club which saw Rynhardt Elstadt bounce off Lowe and Rob Kearney make an important tackle on Cheslin Kolbe.
Charlie Faumuina was pinged for not rolling away five minutes later, allowing Sexton to draw the hosts level in front of a 42,960-strong crowd.
Following an overcooked restart from Ramos, Leinster's attack ignited as they rumbled through nine phases.
Their running angles and power in the carry got them over the gain-line and Sean O'Brien set up winger Lowe to break Pita Ahki's tackle and score in the left corner past Elstadt.
South African speedster Kolbe threatened from a kick chase and in Toulouse's eagerness to attack from deep, a Ramos kick was charged down by Henshaw.
The ensuing pressure saw Gray binned for a sly hand at a ruck, and Leinster duly punished him with a well-executed line-out maul sending McGrath over for a well-taken 25th-minute try.
It was 14 men apiece after Henshaw's deliberate knock-on right on the Leinster line as Toulouse stormed back, with referee Wayne Barnes opted against the penalty try despite prop Faumuina being poised to score from the pass.
Ramos' close-range kick was the final score of the opening half with Leinster missing out on a late Lowe try due to Jack Conan's obstruction in the build-up.
Ramos' right boot punished a Garry Ringrose high tackle on the restart but the hosts hit back after good work from Jordan Larmour and Cian Healy put flanker Fardy stretching over for a seven-pointer near the right corner.
Ntamack landed a penalty, with his introduction moving the dangerous Antoine Dupont to scrum half. But that kick was cancelled out in the 65th minute by Sexton in his final act, with Elstadt the guilty party for an off-the-ball tackle.
The Top 14 outfit continued to be foiled by Leinster's resilient defence, this time Larmour swooping in to intercept close to his line and deny Yoann Huget a try.
The visitors' race was run and with Ringrose finding gaps and Lowe agonisingly close to a deserved second try, it was left to replacement fly-half Byrne to boot the title holders to an 18-point winning margin.