The Giants booted eight-straight goals from midway through the first quarter to late in the second term to blow Saturday's game at the MCG wide open.
St Kilda twice managed to come back from more than 40 points down, even cutting the margin to just 19 points early in the last term.
But GWS' early lead was too much for the Saints to overcome as the Giants ran out winners 15.11 (101) to 11.11 (77) in rookie coach Adam Kingsley's first final in charge.
The No.1 defence in the AFL during the home-and-away season, St Kilda, in their first final in Melbourne since 2011, gave up a score of more than 100 for just the second time this year.
GWS' total was their second best at the MCG in club history, just falling short of the 103 they scored in a loss against Melbourne in 2013.
The Giants will meet the loser of the Brisbane Lions and Port Adelaide clash in an away semi-final next Saturday night.
After finishing 16th during a difficult campaign last year when long-time coach Leon Cameron departed, the Giants drew on their experience of making a grand final in 2019 and playing in two post-season games in 2021.
GWS first made the finals in 2016 and have won at least one match in all six of their September campaigns.
Classy left-footer Josh Kelly had a final to remember, finishing with 27 disposals and two goals to be the most influential player on the ground.
Tom Green (36 possessions) and Lachie Whitfield (31) also enjoyed plenty of the ball as GWS had more drive out of their midfield than St Kilda.
The Giants had five individual goal-kickers, led by Jake Riccardi (three) while their defence stood tall to counter-attack from numerous dangerous forward entries from St Kilda.
Saints captain Jack Steele stood tall with 38 possessions, while Max King (three goals) and Cooper Sharman (two) both had threatening periods up forward.
Both teams were hit by late outs just before the sudden-death final.
St Kilda were first, announcing the withdrawal of Tim Membrey due to a personal health matter.
The experienced forward had been poised to line up in attack alongside fellow tall Max King for just the third time this season.
Just an hour before the bounce, star GWS midfielder Stephen Coniglio was pulled out after suffering an eye injury at training during the week.
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