A week after coming from 44 points down to beat GWS, the Lions erased a 25-point third-quarter deficit in their 14.11 (95) to 12.13 (85) victory at the MCG on Saturday.
Chris Fagan's side will meet Sydney on September 28 in the first grand final featuring two non-Victorian clubs since 2006.
It will be Brisbane's second successive grand final appearance, following a heartbreaking four-point defeat to Collingwood last year.
The Lions, who finished fifth on the ladder, could become just the second team to win the premiership from outside the top four under the current finals system, introduced in 2000.
Geelong, meanwhile, will rue another missed opportunity after losing a preliminary final for the sixth time in Chris Scott's 14-year coaching tenure.
The Cats lost Max Holmes to another hamstring injury, two years after the speedster missed the 2022 premiership with a minor strain.
Captain Patrick Dangerfield would also have been in doubt for the grand final if Geelong had qualified, facing scrutiny over a dangerous tackle on Hugh McCluggage.
Instead it is McInerney who shapes as Brisbane's heartbreak story after hurting his left shoulder late in the first quarter.
The ruck star carried on bravely but was substituted out of the game during the third quarter and taken to hospital.
Lions co-captain Lachie Neale was huge for his side with 31 disposals and 12 clearances, lifting during the third quarter with the game on the line.
Geelong led by 25 points early in the third term, but Brisbane kicked seven of the next eight goals and prevailed in a gripping finish.
Callum Ah Chee kicked three goals for the Lions, including two crucial majors in the final term.
His snap with less than three minutes left gave Brisbane the lead for the final time, before Cam Rayner sealed the result with a left-foot shot on the run.
Joe Daniher was also important for the Lions, stepping into the ruck when McInerney went down, while Ollie Henry kicked a game-high four goals for Geelong in a losing cause.
