Mt Albert looked to have the game won when Dillon Rota gathered a loose bomb to score with a minute to go, but Walker's conversion sailed just left of the uprights to ensure the Fox would stay with the Pirates for another year.
After finishing the season as minor premiers Pt Chevalier came into the match as underdogs, and had lost to the Lions twice in the past month.
The grand final win means the Pirates now hold all of the major Auckland Rugby League trophies - the Fox, Konica Minolta Roope Rooster, Stormont Shield, Kiwi Shield and Rukutai Shield.
Both sides then had numerous chances to seal the game in the dying minutes but an inability to get field goal attempts away meant the game required extra time.
Current Premiership rules mean that two five-minute periods of extra time must be played before golden point comes into effect.
In the end Pt Chevalier came up with the clutch plays when they counted, with Carl's 86th-minute sideline conversion, following Solomona's try, proving the decisive play.
Pt Chevalier 18 (Malua, Solomona 2 tries; Carl 3 goals).
Mt Albert 17 (Faaumuumu, Houma, Rota tries; Walker 2 goals; Walker 1 field goal).