Campion skipper Lucy Allan put her side ahead 15 minutes into the second half.
As United piled on the pressure, they were awarded a corner with time almost up.
Kim-Maree Larby swung the ball towards the far post, where teammate Stella Perano headed it goalwards. After a goalmouth scramble, Amelia Paull levelled the scores.
In extra time, Dara Mulrooney ran on to a loose ball and blasted her shot through the keeper's hands to put Campion 2-1 up.
Despite United's best efforts on tired legs, they couldn't draw level again, Knight said.
“It was an awesome effort from our girls to go two hours and never give up,” she said.
“Well done to Campion on their season and good luck for the final.”
Campion play Bohemians in the cup final at Childers Road Reserve at 10am on Sunday.
Campion clinched the league by beating runners-up Wainui Riverina 1-0 on August 7.