Northampton enjoyed the lion's share of the territory early on and were rewarded with a penalty from their out-half Stephen Myler after 11 minutes.
It was the jolt the hosts needed and from the restart, Jamie Hagan linked well with the returning Luke Fitzgerald and their efforts reaped a penalty. Ian Madigan's kick found touch metres from the line but Leinster's threat was quelled.
Almost immediately the Saints roared for the corner after a fine Paul Diggin break and his pass eventually found his opposite winger Noah Cato, who was superbly tackled into touch by Andrew Conway.
With debutant Sean Cronin keen to get his hands on the ball and Madigan pulling the strings at out-half, Leinster began to assert themselves and a bout of pressure, inches from the Saints line, almost resulted in the first five-pointer of the bright evening.
However, Myler doubled his side's advantage in the 25th minute with his second penalty to make it 6-0. The game ebbed and flowed and Leinster skipper Rhys Ruddock was a towering presence in the loose.
But it was the Saints who maintained their territorial pressure and Myler slotted home his third successful kick in the 36th minute to give the English side a 9-0 half-time lead.
Joe Schmidt's men finished the opening half on a high but despite separate breaks from the likes of Kevin McLaughlin, man-of-the-match Dominic Ryan and then Fitzgerald, they could not breach the Saints rearguard and they went into the interval nine points adrift.
The second half continued in a similar vein and a series of substitutions around the 50 minute mark enabled first seasonal appearances for the likes of David Kearney.
New Zealander Mat Berquist slotted home the points to reducee the deficit to 16-7, and the game was right back in the melting pot.
Lamb added a late penalty though to seal the away win, before the industrious Carr sprinted clear of the Saints defence to score a deserved try.
LEINSTER: Andrew Conway; Fionn Carr, Brendan Macken, Eamonn Sheridan, Luke Fitzgerald; Ian Madigan, Isaac Boss; Heinke van der Merwe, Sean Cronin, Jamie Hagan, Damian Browne, Steven Sykes, Kevin McLaughlin, Dominic Ryan, Rhys Ruddock (capt).
Replacements used: Devin Toner for Sykes (29 mins), Shane Jennings for McLaughlin (half-time), David Kearney for Fitzgerald (50), Mat Berquist for Madigan, John Cooney for Boss, Aaron Dundon for Cronin, Mark Flanagan for Browne (all 51), Leo Auva'a for Ryan (61), Darren Hudson for Sheridan (63), Ian Madigan for Conway (66), Jason Harris-Wright for Dundon (73), Cillian Willis for Madigan (76). Not used: Jack McGrath, Jack O'Connell.
NORTHAMPTON SAINTS: Greig Tonks; Paul Diggin, Tom May, James Downey, Noah Cato; Stephen Myler, Lee Dickson; Paul Doran Jones, Brett Sharman, Brian Mujati, James Craig, Mark Sorenson, Calum Clark (capt), Ben Nutley, Adam Eustace.
Replacements: Mike Haywood, Alex Waller, Tom Mercey, Martin Roberts, Tom Hicks, Ryan Lamb, Scott Armstrong, Ben Prescott, Andy Long, Stuart Commins, Cesar Sempere, Charlie Sadler, Sam Farmer.
Referee: George Clancy (IRFU)