The hosts began with eight straight behinds and then, with the game at their mercy, kicked nine consecutive points and another out on the full in a tense Sunday night high-stakes battle.
But Hugh McCluggage and Darcy Wilmot's brilliance told as the hosts won 11.23 (89) to 11.13 (79) to lock in third spot.
The Hawks would have shot to fourth with victory but will instead begin the finals from seventh or eighth, depending on the result of Wednesday's last home and away game between current ninth-placed side Gold Coast and Essendon.
Jack Ginnivan (23 disposals, two goals) was brilliant for the visitors, his nerveless set shot with seven minutes to play making it a two-point game.
But Brisbane did just enough, Bruce Reville snapping from the pocket and McClugguge (32 touches, two goals, three assists) setting up Cam Rayner for the goal that finally broke the Hawks.
The Lions' season-long accuracy concerns at the Gabba, where they were 6-4 before Sunday, didn't let up.
Charlie Cameron (two goals), Zac Bailey and Sam Day all hit the post as Brisbane mustered just eight behinds in a goalless first quarter.
Mabior Chol didn't have the same issues at the other end, marking and kicking truly three times as the visitors backed their swift ball movement to open up the hosts' defence.
McCluggage ended the pain with an early second-quarter goal that was answered by a pin-point Jack Gunston (two goals).
Brisbane then briefly hit top gear, Jaspa Fletcher scoring from a stoppage, Kai Lohmann kicking two in a row in his latest injury return, and Cameron accurate from his set shot.
Logan Morris kicked a goal off the deck to begin the second half and when Cameron snapped his second the hosts were threatening to rip the game open.
Again they would rue their misses, with the Lions kicking nine straight behinds and another out on the full in a tedious third term.
Hawthorn scored 2.3, with Jack Gunston and Lloyd Meek accurate and with Karl Amon's roost from outside the arc also on target but beaten by the siren.
The margin was back to five when the Hawks struck first after the last break.
McCluggage again broke the drought with a goal earned by brave centre corridor movement, while Wilmot (27 touches, eight rebound 50s) was electric and clean in defence.
Ginnivan's snap hit the post but he struck his set shot moments later to make it a two-point game with seven minutes to play.
But the Lions pushed back, Reville curling through a snap after Amon was pinged for a deliberate out-of-bounds handball deep in his defence.
McCluggage again created for his team, forcing the ball forward for Rayner to chip through a crucial goal before Nick Watson walked in a Hawks reply to set up a nervy finish.
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