Galappththi posted a handy 18 runs while Douglas was the last man to be dismissed with 24 runs in the book as Cadets were bowled out for a very gettable 142.
Long-serving Mount bowlers Tony Goodin and Dale Swan both finished their spells with three wicket bags while all-rounder Peter Drysdale returned 2/13.
Mount Maunganui made their intention clear from the start of their reply, with their first wicket falling at the 46 run mark when Ben Pomare was bowled for 26.
Ben Musgrave kept the pressure on with opener Peter Drysdale to reach 31. However, the Mount innings belonged to Drysdale, who anchored his sides six wicket Williams Cup victory with an unbeaten 59 off 89 balls.
Mount Maunganui captain Matt Golding said: "Our keys to success was the overall strength of our team being able to bat right down the order and also having at least seven very good bowlers.
"Our standout players were every bowler, being asked to bowl first on a flat track, they did very well and it was a whole team effort. Pete (Drysdale) anchored the innings and Ben (Musgrave) played a match winning knock."
The Mount Maunganui win added the Williams Cup to their Baywide T20 Championship and also gave them the Hart Family Trophy Baywide Champion of Champions title for the 10th time.
"It was a great way to finish the season, winning the Williams Cup, which sums up our season and what we have achieved," Golding said.
"It is awesome to send Nick Smith into retirement with a Williams Cup win. He has contributed a lot to the club and we will always grateful for it and there was no better way to send him off than winning the 'Old Bill'."
Flying Mullet Te Puke, finished the Baywide season with the Versatile Tauranga Williams Cup Plate after beating Holland Becket Law Tauranga Boys' College by three wickets at the Te Puke Domain.
Tauranga Boys' batted first and were bowled out for 196. Isaac White top scored with 46 with Liam Collett posting 39 not out.
Mikare Leef was the best of the home sides bowling attack taking three wickets for 27 runs.
Devon Surrurier was in grand touch, anchoring the Te Puke innings with 94 runs at a run a ball. Blair McKenzie belted a neat half century with Jamie Oakley chipping in with 32 runs.
Williams Cup Final
Cadets 142 (Chris Mascall 24, Taylor Douglas 24, Fergus Lellman 22; Dale Swan 3/20, Tony Goodin 3/33, Peter Drysdale 2/13) lost to Mount Maunganui 145/4 (Peter Drysdale 59no, Ben Musgrave 31, B Pomare 26; Tim Pringle 2/34).
Williams Cup Plate
Tauranga Boys' College 196 (Isaac White 46, Liam Collett 39no, Blake Pentecost 26, Charles Williams 25; Mikaere Leef 3/27) lost to Te Puke 198/7 (Devon Surrurier 94, Blair McKenzie 50, Jamie Oakley 32; Blake Pentecost 3/15, Charles Williams 3/38).
