Victory means the Irish province will have the chance to clinch a tournament-record sixth European crown when they face English champions Saracens, unbeaten in Europe this season, in the May 11 final in Newcastle.
Leinster won the trophy last season and Saracens the lifted it the two seasons before that. The final at St James' Park is also repeat of last season's quarter-final when Leinster ended Saracens' bid for a third straight European crown.
Leinster outscored French league leaders Toulouse three tries to none on Sunday, with the visitors having just four penalties to show for their efforts.
Flyhalf Sexton, far from his best as Ireland lost their Six Nations title this season, was playing his first match since facing Wales in Cardiff on March 16. He had been out with a thigh injury.
Encouragingly for Irish hopes at this year's World Cup in Japan, he looked somewhere near his best both as a goalkicker and in open play.
Fullback Thomas Ramos opened the scoring in the fifth minute only for Sexton to draw Leinster level soon afterwards in front of a crowd of more than 42,000.
Leinster took the lead when Sean O'Brien found James Lowe who broke a tackle from Pita Akhi to score in the left corner. It was the winger's sixth try in eight European appearances.
Both sides suffered a first-half sin-binnings.
A hand in the ruck by Toulouse lock Richie Gray from the ensuing play saw him yellow carded.
With Toulouse down to 14, Luke McGrath scored from a maul.
Then Henshaw's deliberate knock-on made it 14-a-side, with experienced English referee Wayne Barnes deciding against awarding a penalty try even though prop Charlie Faumuina was set to score from the pass.
Ramos' penalty double kept Toulouse in touch, with Leinster denied another Lowe try due to obstruction by Jack Conan in the build-up.
Sexton's kicking, meant Leinster led 17-6 at half-time in what was effectively a home game for the Dublin-based side.
Leinster had absorbed the loss of Rhys Ruddock for fell ill pre-match, and his replacement Australian backrow Scott Fardy crossed for the champions' third try in the 52nd minute.
Ramos edged Toulouse - like Leinster four-times kings of Europe - closer before Jordan Larmour and Cian Healy sent Fardy, stretching over the line, for a converted try near the right corner.
Toulouse replacement Romain Ntamack kicked a penalty but that was nullified by Sexton in his final act of the game before he went off in the 65th minute.
Leinster were now in complete command, with Ross Byrne, Sexton's replacement, adding a penalty.