The Cats dynamo kicked five goals and had 30 disposals - both career-highs - as the visitors started hot, weathered the Lions' reply, then found another gear in a 17.15 (117) to 11.10 (76) win on Thursday night.
Mannagh, 28, is in just his third AFL season after finally winning a contract with the Cats and debuting as a 26-year-old after years of VFL dominance with Werribee.
"It was a big game … in the slippery conditions there are some guys who look a bit more at home," Cats coach Chris Scott said.
"It was a high-level game in terms of dangerous small forwards around goal - from both teams - and he was the outstanding one of those.
"It's a great story ... he's 28 but it feels he's really early in his journey."
There were shades of last year's MCG decider, only with roles reversed, as the Cats ripped the Lions to shreds in a dominant third quarter to set up the win and improve to 7-3.
Brisbane, who had won six of their previous seven games, dropped to 6-4.
"It would be a quick reaction to have deep concerns," Lions coach Chris Fagan said, admitting many of his stars "didn't have good nights".
"Did we have an off night? I think Geelong had an on night.
"There were lots of areas down on usual standards, but you can put that down to their hunger and hunt on the foot and pressure on us."
Bailey Smith had 34 touches and 11 tackles to go with seven clearances and two goals, while Tom Stewart (17 intercept possessions) was superb at the back, setting the tone for his side's stifling, workaholic defence.
Darcy Wilmot had 31 disposals and Charlie Cameron was lively with three goals to lead the Lions, but Hugh McCluggage was uncharacteristically sloppy to sum up Brisbane's fortunes.
Veteran Lion Dayne Zorko, who has missed time with a calf injury this season, was a late withdrawal as Chris Fagan instead promoted James Tunstill on a wet Brisbane night.
The Lions' kicking was horrid early and Geelong dominated the stoppages, Oisin Mullin keeping Lachie Neale quiet as they won seven of the first eight clearances.
Patrick Dangerfield and Smith both kicked impressive goals from the right pocket as the Cats collected 23 inside-50s to eight and led by 22 points.
The hosts found a response though and, behind a 15-3 clearance count, roared back to level when Cameron chased Sam Draper's go-ahead kick and rolled it home from that same right pocket.
But a Darcy Gardiner 50m penalty for late contact took some wind out of their sails, and more turnovers led to two late Geelong goals and a 12-point halftime lead.
Logan Morris missed two set shots. and when Zac Bailey dropped a mark inside 50 the Cats put their foot down, booting five goals in seven minutes to stretch the lead to 41 points.
Mannagh managed three of his own in a five-minute stretch before Jeremy Cameron broke free to snap a goal and cancel out Draper's streak-snapping goal for the Lions.
Brisbane maintained hope with two late goals - they hit the post twice too - late in the term to make it a 29-point game at the final break.
But Cameron's third major, after more Brisbane fumbles, pushed the Cats beyond 100 points and into safe waters.
"It's hard not to be content but still hard to get a read and my guess is it's close and we're doing what we can to chase their level," Scott said.
"We'll get better, how that looks compared to the rest of the competition is a bit hard to work out."
Jack Martin (calf) was kept off in the final quarter as a precaution but Scott said he could have returned to play if needed.
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