GWS rallied from an awful start, conceding a goal after just 20 seconds and the first 19 points, to notch a 17.9 (111) to 13.7 (85) win at Engie Stadium on Saturday.
It was the Giants' fourth straight win - exceeding 100 points each time - and their fourth consecutive victory over the Cats.
Geelong cut a 20-point deficit in the third quarter back to seven, but Cadman turned the game after an uneventful first half in which he logged just one behind.
He kicked four goals in the third term as GWS opened up.a 25-point buffer going into the last quarter.
Cadman added two more in the fourth, displaying a strong aerial presence and accuracy from both set shots and general play.
He eclipsed his previous career best of five goals against Brisbane last month.
"I think he has been building for a long while, now he's probably getting the rewards for all that effort in terms of goals," GWS coach Adam Kingsley said of Cadman.
Jake Stringer added four goals, his best tally for the Giants, with three of them In the first half, though Jessie Hogan was kept to one major.
"I don't think they (Hogan and Stringer) get enough credit, it obviously doesn't go on the stat sheet, but I think Jessie created probably three or four of my goals," Cadman told AAP.
The Cats' early pressure forced turnovers around their attacking 50 and they punished the Giants on the scoreboard.
The shellshocked Giants worked their way into the game to trail by 13 points at the first change, then exploded out of the blocks in the second quarter.
GWS booted the first five goals of the term, dominating clearances and inside-50s to earn a 20-point lead.
"Hats off to the mids, they got the ball going forward," Cadman said.
Geelong rallied before halftime, kicking the last two goals to trail by just six at the major break.
The game took another twist early in the third quarter as GWS booted the first two goals to swell their advantage to 18 points.
Geelong surged to get within seven, but Cadman converted a strong mark into a set-shot goal, then landed an impressive snap.
Former GWS favourite and the club's all-time leading goalkicker, Jeremy Cameron, had a quiet return in his first game at the ground since leaving the Giants after the 2020 season, kicking 1.2.
The Geelong spearhead and Coleman Medal leader had just four first-half touches and spent time being treated on the bench in the second quarter.
Geelong coach Chris Scott said while Cameron and Patrick Dangerfield were both restricted and Tom Stewart suffered an ankle tweak, those were not reasons for the loss and he thought his team played poorly.
"It felt like for the last three quarters we were under the pump but were still in front in inside-50s," Scott said.
"At the other end the quality of their entries was better than ours, and again a lot of the time that was bad turnovers, which leave you exposed in an open back half."
In his 250th game, GWS half-back Lachie Whitfield gathered a match-high 32 possessions and 11 marks, with Finn Callaghan, Harry Rowston and Kieren Briggs also strong for the Giants.
Kingsley said stalwarts Stephen Coniglio and Sam Taylor could be back for Thursday's match against Essendon, but wouldn't guarantee Stringer would back up against his former club off a five-day break.
Shannon Neale was a strong target up forward for the Cats, kicking four goals.
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