Peterhead were seconds away from clinching consecutive promotions only for their hopes to be dashed with a goal which levelled the tie on aggregate.
Peterhaed took the lead in the fifth minute. Martin Wood sent Martin Bavidge clear, he bore down on goal and scored with a thunderous shot high into the far corner from the right-hand side of the penalty area.
The Jags were level three minutes, Mark Roberts chesting the ball down before netting from close range.
Peterhead had opportunities to score before the break, the best falling to Keith Gibson who shot over.
The best opportunity in the second half fell to Peterhead. Chris Hegarty shot wide, Graeme Sharp drove an effort over the crossbar, before Sharp crossed for Wood who shot against the left-hand post.
Two minutes later Bavidge hit the right-hand post, a chance the home side were to rue.
Two minutes into added time a Billy Gibson free-kick struck the defensive wall and deflected into the right corner of the net to send the game into extra-time.
The additional 30 minutes failed to produce a goal and the game went to penalties, which Partick won 4-2 with Paul Ritchie scoring the decisive spot-kick.