The Blues kicked the preliminary final's opening five goals to shock the Gabba faithful in the first quarter but managed just four more in an 11.13 (79) to 9.9 (63) Lions win.
Collingwood await the Lions at the MCG next Saturday in a battle of the regular season's top two teams.
It will be Brisbane's first grand final since 2004 as they aim to break a 20-year flag drought, their 2003 triumph the final chapter of a three-peat that included two wins against the Magpies.
Their Saturday evening triumph ended the Blues' brave run - from 15th in round 13 with just four wins - under coach Michael Voss, the captain of Brisbane's triple-premiership winners.
Josh Dunkley (23 disposals) held Patrick Cripps to 13 touches and two goals, while Sam Walsh led all-comers with 34 disposals.
Jarrod Berry (24 touches) did a mountain of grunt work for the Lions, ruckman Oscar McInerney helped turn the tide and Keidean Coleman (11 intercept possessions, seven score involvements) was arguably best on ground.
Fagan hosed down any injury concerns for star midfielder Lachie Neale, who was left clutching his shoulder after a third-quarter challenge, and applauded his backline for turning the game in their favour.
"It's a good feeling, I've never seen the (dressing) rooms like that ... just a lot of happiness in the room," he said after snapping a run of three preliminary finals losses in the last four years.
"We'll have to settle it down pretty quickly, but enjoy that moment.
"They're hard things to get to, as we know, and it was looking a little bit dicey at quarter-time."
There were ominous signs for Brisbane early when Berry picked out Callum Ah Chee in front of goals, only to cop the club's first free kick for running too far all season.
Matthew Cottrell ran into an open goal at the other end, the first of five straight Blues majors to light up the huge travelling army that had headed north for the clash.
Carlton dominated centre clearances and had 22 inside-50s to the Lions' 11 in that term, Harry McKay and Curnow both flexing after kicking straight in the perfect start.
It took Brisbane until the final 90 seconds of the quarter to string a clean passage together, Eric Hipwood the benefactor with a crucial goal before the break.
Cam Rayner then set the tone with a barnstorming second quarter, while Lincoln McCarthy (two goals, seven tackles) brought the pressure.
Defenders Conor McKenna and Ryan Lester chipped in with snapped goals and the lead could have been bigger if Charlie Cameron had taken any one of his three opportunities in the final minutes of the half.
McInerney then kicked truly after earning a 50m penalty from the first bounce of the second half, Ah Chee making it a 21-point lead late in the third to provide some breathing space.
McCarthy settled things further but goals to Cripps and McKay made it a 16-point game with eight minutes to play.
Brisbane, despite hitting the post twice, steadied again through McCarthy to maintain a perfect record from 13 Gabba games this season.
"What changed was that we lost some critical contests, (lost control of the) centre square bounce and that changed the shape of the game a bit," a proud but disappointed Voss said.
"And we just weren't able to get that ascendancy back."
