Unfancied Saints had looked all set for an against-the-odds win of their own at the midway point thanks to scores from Phil Dowson, Ben Foden and Dylan Hartley, and seven points from the boot of Stephen Myler.
But they had little answer for Leinster - and man-of-the-match Sexton - after the break as they lit up the Millennium Stadium with 27 points in 26 minutes to turn the tie on its head.
Sexton has come in for some criticism for his suspect kicking during previous RBS 6 Nations campaigns but the 25-year-old was the undisputed star of the show in Cardiff.
The No. 10 scored twice in his side's unstoppable purple patch, Nathan Hines added another, and kicked three conversions and four penalties.