In Simon Easterby's first game in charge, the Welsh region came flying out of the blocks after the restart with Fenby and North crossing twice and Kieran Murphy also going over to concluded a seven-try demolition.
The Heineken Cup champions were missing 11 internationals with seven players also out injured and it showed even though Devin Toner and Madigan scored consolation tries.
Coming into the match the Scarlets had not beaten Leinster since the first match of the 2009-10 season but the Welsh side set about busily trying to rectify that unwanted statistic when Scott Williams went over for the first try of the game.
Leinster fly-half Madigan had put the Irish visitors into an early three-point lead after slotting a penalty from deep inside the Scarlets half after four minutes.
But once Scarlets got their talons into Leinster's 22 they made good use of the territory and bravely turned down a kick at goal to boot the ball into the corner.
Scarlets sustained the pressure and from an ensuing five-metre scrum, Williams got into position before receiving the ball and pouncing to burst through between the posts and touching down with Aled Thomas coolly converting.
The hosts began to dominate and their possession play paid off with Liam Williams going it alone to cross the whitewash after 27 minutes after accepting a scoring pass from his namesake Scott.
Thomas again made the conversion to put Scarlets 14-3 up but Leinster hit back with a Madigan penalty just before the break to reduce the deficit to 14-6
The second half was barely two minutes old when North went over in the right corner after Thomas had been stopped inches short of the line, Liam Williams starting the move with a break from defence across almost the whole pitch.
Thomas missed the conversion to keep the scores at 19-6 but minutes later Aaron Shingler picked up the ball and charged to the 22 before offloading to Fenby who went over in the left corner to secure the bonus point.
This time Thomas made no mistake with the boot, boosting his side's lead to 20 points with only 49 minutes gone.
Just before the hour Leinster won a penalty in a kickable position but went for touch, with Toner bundling over the line soon after and Madigan converting to make it 26-13.
Any hopes of a comeback were put to bed when Murphy went over five minutes later, with North racing clear down the wing to add another with six minutes to go.
Scarlets were not done though with Fenby accelerating past the Leinster defence before strolling home for his second try less than two minutes later with Priestland kicking the extras for 45-13 before Madigan scored a late consolation try and kicked the conversion himself.
|Scarlets Score Card|
|Leinster Rugby Score Card|