They defeated reigning champion Scarlets at Dublin's Aviva Stadium to cap a remarkable season for Irish rugby.
It was the first time a Pro14 side had completed the European and league domestic double.
And after Joe Schmidt's national side won the Grand Slam, Leinster have continued the Irish success story.
Tries from Devin Toner, James Lowe, Sean Cronin, Jordan Larmour and Jack Conan and 13 points from the boot of mercurial man-of-the-match Johnny Sexton sealed the win.
Scarlets wing Johnny McNicholl scored a stunning hat-trick, with two late tries from the Welsh region reducing the deficit.
It was a ninth successive win for Leo Cullen's side at the Aviva Stadium as Leinster were crowned league champions for the fifth time.
The victory represented a fitting finale for Leinster's retiring captain Isa Nacewa who was forced off the field in the first half.
For Scarlets, it was a second recent defeat at the hands of Leinster at the Aviva Stadium after their 38-16 Champions Cup semi-final loss in April.
The match started frantically with Scarlets absorbing pressure before a Gareth Davies chip was almost latched on to by Steff Evans.
Leinster rallied and opened the scoring in the sixth minute with a Sexton penalty before that was cancelled out by Halfpenny.
A poor Rob Kearney kick ensured Leinster were offside and Halfpenny punished the Irish province with another penalty.
Kearney made amends with a towering take from a Sexton high kick that galvanised the Dubliners and set the platform for a second Sexton penalty.
Captain Nacewa, who was playing his 185th and final appearance for the Irish province for retiring, was forced off the field in the 19th minute.
Nacewa had been struggling with injury after being replaced at half-time in the semi-final win over Munster.
Sexton was also hurt after a shuddering tackle from Scott Williams but stayed on and gave Leinster the lead with a scruffy penalty.
Ireland number eight Conan launched another wave of Leinster relentless attack with a storming break and the Scarlets' defence had to crack under the pressure with lock Toner crashing over. Sexton missed the conversion.
Scarlets struck back with McNicholl involved in the build-up and eventually poached a try after Gareth Davies was stopped inches short.
Wales flanker Aaron Shingler received lengthy treatment on a leg injury after a storming break but could not continue and was replaced by Will Boyde.
The Leinster cross-kick tactic continued to pay dividends when Sexton found James Lowe on the left flank. The Welsh region were forced to concede penalties and Lowe crossed in the left hand corner from a driving lineout.
Sexton's conversion gave Leinster a 21-11 interval lead in what proved a disastrous end to the half for the Scarlets.
Leinster continued their attacking onslaught in the second half as they dominated possession and territory with only a brilliant Scott Williams steal stopping a third try.
Sexton was unnerved by a Williams tackle that was looked at by Scottish television match official Neil Paterson and South African referee Stuart Berry and the Wales centre was penalised.
Sexton produced a magical touch-finder from the resulting penalty and Ireland hooker Cronin dived off with sheer elation. Sexton converted.
A wayward Williams chip allowed Larmour's inspired individual try, which saw him collect the ball, kick ahead and produce a Hollywood style pick-up to score.
McNicholl scored a fine second try in the right-hand corner before a flowing Leinster move provided a fifth try with Conan cantering over.
Scarlets rallied with two late tries from replacement prop Werner Kruger and a third for McNicholl to give the scoreline more respectability.
But there was no doubting who deserved the victory as Leinster celebrated.
Leinster: R Kearney; J Larmour, G Ringrose, I Nacewa (capt), J Lowe; J Sexton, L McGrath; C Healy, S Cronin, T Furlong; D Toner, J Ryan; R Ruddock, D Leavy, J Conan.
Replacements: J Tracey, J McGrath, A Porter, S Fardy, J Murphy, N McCarthy, J Carberry, R O'Loughlin.
Scarlets: L Halfpenny; J McNicholl, S Williams, H Parkes, S Evans; R Patchell, G Davies; R Evans, K Owens (Capt), S Lee, L Rawlins, S Cummins, A Shingler, J Davies, T Beirne.
Replacements: R Elias, W Jones, W Kruger, D Bulbring, W Boyde, J Evans, D Jones, T Prydie.
Referee: Stuart Berry (South Africa)
Assistant referees: Marius Mitrea (Italy) and Mike Adamson (Scotland)
TMO: Neil Paterson (Scotland)