The Irish side were cut adrift at half-time, 22-6 down and with precious little hope of dragging the Saints back, but dominated the second period to run out 33-22 winners.
Ireland fly-half Sexton was the central figure in the dramatic turnaround, personally responsible for 28 points thanks to two tries, four penalties and three conversions.
In doing so he fell just two points short of Diego Dominguez's record points tally from the 2001 final.
For Leinster it was the second European crown in three years, with their third and fourth still to come in 2012 and 2018.
Northampton, captained by future England skipper Dylan Hartley, were left distraught after finishing the season empty-handed. Just seven days earlier they had been dreaming of landing the double only to be beaten by Leicester in the Premiership semi-finals.
Brian O'Driscoll later credited Sexton with an inspirational team talk during the interval, but the man himself was happy to downplay his oratory.
"I'm a bit of a nerd when it comes to sport and I just said that we see in sport that teams can come back like Liverpool (in the 2005 Champions League final) a few years ago," he said.
"Stuff like this happens. We had to believe and we took our chances."
One year on, Leinster found a much simpler way to retain the cup, hammering Ulster 42-14 at Twickenham.