After winning the toss and electing to bat, Halls Head posted a competitive total of 8-195 off its 50 overs, largely thanks to a half-century from Jack Manual (56).
Joe Ducker (3-22) and Gehan Fernando (3-39) were the best with the ball from Mandurah.
A solid opening partnership from Brendan Ingle and Paul Okey (both 38) got Mandurah off to a flying start, however it was Ben Drummond who cemented the win with an unbeaten 73.
Mandurah finished its innings 5-198 after 50 overs.
Elsewhere, Waroona demolished Pinjarra by 10 wickets after rolling the home side for just 84.
Ben Wright boasted enviable figures of 4-1 off 2.5 overs.
Second-placed Waroona then went to work at the crease, demoralising Pinjarra by smashing 9-262 off 50 overs.
Stephen Mason top-scored with 76 in a crucial win for the away side, which is looking to sew up second spot from a chasing Shoalwater Bay.
South Mandurah cruised past bottom-placed Hillman, which posted 118 when sent in to bat.
Matt Hill was the chief destroyer for Souths, claiming 5-21 off eight overs.
The visitors posted 199 in response to the Hornets’ total to claim the points and keep in touch with the top four with just one game of the regular season to go.
Elsewhere, Shoalwater Bay defeated Singleton Irwinians to return to third, thanks to 68 from Brandon Burt.
Baldivis White Knights succumbed to Warnbro in a match for nothing more than pride at the wrong end of the table.