The Dublin based club secured a 40-32 victory over Scarlets on Saturday to claim a first league title since 2014 and the success comes swiftly off the back of their European Rugby Champions Cup triumph over Racing 92 earlier this month.
Indeed, Leinster have become the first side to win the Pro14 and Europe's top prize in the same season and Cullen, who has masterminded the double success, was quick to pay tribute to his players after Saturday's victory, particularly as they also recorded a win against Munster in between the two finals.
"The last couple of weeks have been a mental and physical test for all of us," admitted Cullen.
"The European game a couple of weeks ago, the feeling at the end was incredible because it was so tight and only going into the lead in the 79th minute of the game.
"Huge challenge last week against Munster in the RDS and dealing with the celebrations etc. try and get our feet back on the ground this week was a challenge, but credit the players. They're the ones that produce the goods on the field and I thought they were exceptional."
Leinster even had to withstand a late flurry from Scarlets to clinch their Pro14 success, but they ultimately kept the Welsh club at bay as they clinched a fifth title to become the most successful club in the competition's history.