Ian Madigan scored Leinster's second in the 21st minute and Sexton and Ronan O'Gara completed the scoring to leave Leinster in front 20-14 at half time.
O'Gara struck the upright with a 48th minute penalty, Sexton landed one the far end and added the extras to Brian O'Driscoll's 52nd minute try.
For some strange reason referee Leighton Hodges went upstairs when Conor Murray scored in the 66th minute and Ian Keatley - on as a 60th minute for O'Gara - kicked a marvellous touchline conversion to put Munster back in contention.
Munster owned the end game. Casey Lualala scored after Keith Earls appeared to have been stopped short but Hodges deemed there'd been a knock on.
Munster : D Hurley; D Howle.tt (capt), K Earls, C Laulala, S Zebo (J Downey).; R O'Gara (I Keatley), C Murray; D Kilcoyne (W du Preez), M Sherry (D Varley), BJ Botha; Donncha O'Callaghan (B Holland), D Ryan; Dave O'Callaghan, S Dougall, P O'Mahony. Replacements : S Archer, P Butler, D Williams,
Leinster: I Madigan; A Conway, B O'Driscoll, F McFadden, I Nacewa; J Sexton, E Reddan; H van der Merwe, R Strauss, M Ross; D Browne, D Toner; K McLaughlin, S Jennings, J Heaslip (capt). Replacements : S Cronin, J McGrath, J Hagan, T Denton, J Murphy, J Cooney, N Reid, F Carr.
Referee: Leighton Hodges (WRU).