Promoted opener Matt Golding posted a handy 33 for the Mount, with Bay of Plenty representative bowler Donovan Deeble taking 4/37.
Mount Maunganui's quick bowlers produced outstanding line and length in the first 10 overs to restrict the Cadets top order to a handful of runs.
The introduction of Mount left arm slow bowler Nick Smith paid immediate dividends, with the spin bowler taking six wickets which was hugely instrumental in bowling out Cadets for 137.
Mount Maunganui quick Alex Gooding was rewarded with three bowling scalps.
Cadets skipper Ben Christensen battled away to top score for his side with a hard fought 28, with Tom Boorman and Donovan Deeble also reaching the twenties.
The Mount Maunganui victory was the fourth time in eight seasons that Mount Maunganui has lifted aloft the long-time Baywide prize in triumph.
The Williams Cup title decider completed the Bay of Plenty cricket season, where the three big premier prizes were shared around.
Greerton annexed the pre-Christmas Baywide Cup, while Rotorua Central took out the Baywide T20 title and Mount Maunganui the Williams Cup which was first competed for in the 1932/33 season.
Williams Cup final:
Carrus Mount Maunganui 265 (Ben Musgrave 79, Ben Guild 65, Matt Golding 33; Donovan Deeble 4/37) defeated Element IMF Cadets 137 (Ben Christensen 28, Tom Boorman 22, D Deeble 21; Nick Smith 6/35, Alex Gooding 3/17)