The Hawks will limp into the finals next week with easily the worst form of any side in the top eight after the Saints won 13.11.89 to 13.9.87.
The victory was the Saints' first since the sacking of Malcolm Blight.
The final siren went as Hall lined up for his fourth goal and team-mates mobbed him after his shot went straight through.
It was the first time St Kilda had been in front all night.
Goals to strong-marker forward John Barker and Kris Barlow within a minute early in the last term gave the Hawks a 23-point buffer and they looked in control despite struggling at times against the lowly Saints.
But Hawthorn could only manage another two behinds for the match, while Hall kicked his third and Brett Voss kicked an improbable goal to pull the Saints to within 10 points.
Stephen Milne then picked up a free in the goalsquare as Hall's long shot for goal missed and the resultant goal put the Saints within four points.
Milne snapped from a pocket, but hit the post, and the Hawks failed to clear the ball out of defence from the kickout.
Milne was in the action again, kicking to Hall for the mark and match-winning goal.
The Saints worked extremely hard in the third quarter and had the ball in their forward 50 for most of the 10 minutes immediately after halftime, but did not kick a goal until 16 minutes into the term.
They reduced the margin from 17 points at the main break to two goals at the last change.
The Saints lost ruck-forward Peter Everitt before the game with a strained calf muscle and they looked in trouble early.
It took eight minutes for Hawthorn's Nathan Thompson to open the scoring in the match with a behind, but the Hawks led by 26 points at quarter time and looked in control.
Milne kicked three goals and Brett Moyle was particularly strong in the midfield during the second half, while Barker kicked three for Hawthorn and Joel Smith worked hard in defence.
Scoreboard:
ST KILDA: 3.1, 7.4, 9.7, 13.11 (89)
HAWTHORN: 7.3, 10.3, 11.7, 13.9 (87)
Goals: St Kilda: B Hall 4 S Milne 3 J Blake M Capuano F Gehrig J Koschitzke B Moyle B Voss. Hawthorn: J Barker 3 K Barlow 2 M Graham 2 R Vandenberg N Thompson A Lekkas N Lonie R Tallis B Dixon.
Best: St Kilda: M Capuano B Moyle B Hall J Koschitzke J Peckett S Baker. Hawthorn: J Smith S Greene J Barker M Graham J Hay.
Umpires: H Kennedy, K Nicholls, S Hanley.
Official crowd: 24,113 at MCG.
