They went into the semi-final against Wests Scarborough ready to play a intense rugby and a team effort led to a 17-15 win.
The win came down to a great try scored in the final moments of the game.
Good skills from both the backs and forwards set up the winning try.
Mandurah will next play against Wanneroo at Landsdale, kicking off at 11am on Saturday.
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Meadow Springs Sports Facility played host to the seniors Rugby Union grand finals.
The games were a great success, with large crowds supporting the teams throughout the day.