The Thistles made sure of a home success by seeing off the Tigers 33-12 at the Greenyards.
Scotland's national sevens development team raced into a 19-0 lead before a try just before half-time gave the English visitors some hope.
But the Thistles powered to victory in the second half.
They had beaten Ulster 21-5 in their semi-final, while Leicester disposed of Melrose 31-7.
The anniversary tournament was played in front of 12,000 fans who appropriately witnessed 125 tries.
Thistle's Mike Adamson was voted player of the tournament.
Scottish Thistles: Scott Forrest (Ayr)* CAPTAIN, Mike Adamson (Glasgow Warriors), Ben Addison (Stirling County)*, Calum Forrester (GHA)*, Chris Fusaro (Heriot's)**, Greig Laidlaw (Edinburgh), Colin Shaw (Glasgow Warriors), James Thompson (Heriot's)*, Richie Vernon (Glasgow Warriors), Ali Warnock (Edinburgh)
* = Member of Scottish Rugby's Senior National Academy
** = Member of Scottish Rugby's Junior National Academy and supported by Area Institute of Sport