The Lions, who were inaccurate in the first half, came through a Saints' second-half fightback to ensure they'll have two finals at their fortress of a home ground with a 9.18 (72) to 9.6 (60) win.
The loss leaves sixth-placed St Kilda's finals fate up in the air, with the chance of a home elimination final depending on Sunday's results.
Lions forwards Joe Daniher and Charlie Cameron kicked two goals each for the hosts while Tim Membrey had three goals for the Saints.
Lachie Neale led the way with 30 disposals as well as four marks and 10 clearances for the Lions while veteran Dayne Zorko (22 disposals, 12 marks, one goal) was busy throughout.
Saints' ruckman Xavier Marshall finished with 24 disposals, five tackles and 27 hitouts to stand out for the visitors, while Jack Sinclair also racked up 30 disposals to match Neale's count.Â
Brisbane spearhead Daniher set the tempo for a scrappy first half when he somehow shanked a set shot from right in front in the game's opening minutes.
Daniher's miss came with audible groans from the Gabba crowd, as the Lions went on to kick 4.11 in the half, failing to capitalise fully on their dominance of territory and possession.
Daniher did eventually find his 50th goal of the season in the second quarter but the Lions would've gone into the dressing room at halftime knowing a 14-point lead was scant reward for their efforts.
That profligacy was punished when a fired-up Saints emerged for the second half, goals to Brad Crouch and Membrey wiping out the Lions' halftime advantage.
An opportunistic finish by Dan Butler 15 minutes into the quarter put the visitors' ahead for the first time since the early stages of the game.
Just when the Lions needed a spark, they found one in Cam Rayner.
The 23-year-old first surged clear down the wing before a pinpoint pass allowed a sprinting Cameron to level scores, before it was Rayner himself who finished smartly to put the Lions back in front.
Some more quick work by Lions ruck Oscar McInerney eased Brisbane's nerves when he snapped a third straight goal for the home team.
There was a special moment for the Saints when Jack Hayes kicked a goal in his first game of the season after a horror injury run which was celebrated enthusiastically by his teammates and ensured the margin was just one goal at three-quarter time.
Brisbane held firm in the final term though, those 12 extra points proving decisive to ensure they finish the regular season with 11 wins from 11 at the Gabba.
St Kilda were forced to substitute Josh Battle at halftime due to concussion following a collision in the second quarter, but the 24-year-old is unlikely to miss any further football due to the pre-finals bye week.
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