The Mariners, who haven’t made the finals since the 2010-11 season, were led well by their experienced players, including captain Craig Simmons and former coach Brendon Diamanti.
New coach Mark Atkinson will be delighted with the side’s early season results, particularly after being sent in to bat on Saturday.
The Mariners got off to a fast start, with Simmons scoring 40 from 36 balls in typical fashion.
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READ NOWDiamanti scored his second half-century in as many weeks, while indigenous talent Dane Ugle played an entertaining knock of 62 off 57 with three big sixes.
The home side managed to reach a match-winning score of 8-246.
A steady flow of wickets meant Midland–Guildford were never in the chase, with Western Warriors rookie Kyle Gardiner impressing again, taking 3-48.
He was well supported by his spin partner Simmons, who took three wickets as well.
Eventually the away side was bowled out for 189 in the 49th over.
Rockingham-Mandurah remains unbeaten as they look to the challenge of reigning premiers Joondalup and Fremantle in the Saturday-Sunday double next week.