Shoalwater was sent in by Mandurah and posted a modest 9-140 off its 20 overs, thanks to Hammad Khan (33) and Dan Abel (37).
Ian Lattimore did the damage with the ball for Mandurah, claiming 3-13 from his four overs.
Mandurah then got off to a solid start, moving to 48 without loss before Brendan Ingle departed for 10.
It sparked a mini-collapse for Mandurah, losing Jeremy Leatherbarrow for a well-crafted 42, Ben Drummond (3) and Lattimore (4) to be 4-64.
Rhys Donald (21) and Brett Ronan (15) helped Mandurah to 85 before Ronan departed, again leading to a quick succession of dismissals.
Darren Austin (4), David Schonberger (1) and Paul Okey (2) all departed cheaply, leaving Mandurah an impossible task.
The Wyllie Cup minor premiers will be disappointed to finish 2015-16 empty-handed, especially after cruising past Singleton Irwinians in the Premier League semi-final.
Lattimore bashed 54, while Ronan mustered 45 as Mandurah posted 166, a chase that was always going to be difficult for the Irwinians.
Shoalwater also had no trouble in its semi, beating hosts Pinjarra comfortably by 50 runs.
Mitchell Smith (44) and Abel (3-12) were the standouts.
The 2015-16 Peel Cricket Association season has concluded, with the 2016-17 scheduled to start in October.