The Centurions won by 26 runs, amassing 246, with the Tigers falling short on 220 in reply at neutral venue Stevens Reserve in Fremantle.
Talented Joondalup teenagers Jaron Morgan (71) and Clint Hinchliffe (57) combined for a 114-run second-wicket stand to set the base for a solid total.
Tigers bowler James Roberts took a match-high 4-44.
Claremont-Nedlands faced a difficult task when they slumped to 4-96 with the wicket of top scorer Nicholas Hobson (51).
A middle-order rally, led by Tim David (47), gave the team a glimmer of hope but Joondalup remained in control.
Veteran paceman Josh Anderson took 3-41.
The victory gives the Centurions a shot at winning two titles in 2015.
That could have been three had Joondalup not lost to Claremont-Nedlands in the State T20 semi-finals earlier this year.
They are favoured to claim the coveted two-day flag, finishing the season well clear on top of the table.