Sam's the sinner for Saints as red card proves turning point.
In a tense showdown between these two Welsh Premier giants, the tie took a decisive twist 12 minutes from time and, with extra time looming, Jones pounced to deliver the knock-out blow.
The Saints' villain was 20-year-old Sam Finley, a half-time substitute for Aeron Edwards, who showed his exuberance with a two-footed lunge on Chris Roberts that led to an instant red card in the 78th minute.
It handed the initiative to Bangor who cashed in on the numerical advantage to extend the love affair of Neville Powell's side with a competition they have monopolised in recent years.
They will start as firm favourites to lift the trophy for the third time in six years when they face Prestatyn in a derby final.
The best chance fell to the Saints, with a stunning 9th-minute half-volley from Ryan Fraughan producing a brilliant save from Bangor goalkeeper Lee Idzi.
Saints were on a 13-match unbeaten run but Bangor were a much tougher proposition than the last time the two sides met when Saints won 6-0 in a league game.
They had a great 75th-minute chance to move a step closer to the double when a teasing cross from Fraughan looked to have set up top scorer Michael Wilde but Idzi showed immense bravely to nip the ball away from the striker's boot and keep Bangor in the competition.
Jones became the Bangor hero as he dashed Saints hopes of a league and cup double for the second successive season when he latched on to a long ball and superbly lobbed Paul Harrison to send his side into the final.