The early season surprise package were always facing an uphill battle after a poor first day where Willetton claimed first innings points.
Trailing by 17 runs after the first innings, Rockingham-Mandurah needed runs on Saturday to set the hosts a decent total for an outright win.
But the Mariners lost early wickets to slump to 6-101, with First Grade leading scorer Brayden Sutton dismissed for just nine.
Mariners captain Craig Simmons resurrected the innings with 56 from 62 balls, while tail-ender Kyle Gardiner added a handy 33, to set Willetton 179 from 42 overs for an outright victory.
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READ NOWIn a thrilling finale, the Dragons fell just four runs short finishing at 5-175, with Gardiner holding his nerve at the death.
The Mariners teenage spinner continued his great start to the year with 3-54, including the key wicket of Riley Ward late on.
Rockingham-Mandurah host University in a two-day fixture from Saturday 11.40am at Lark Hill next round.