Central Coast booked a two-leg semi-final against Melbourne City, with the first match to be played in Gosford on Saturday afternoon.
Monday night's match at Gosford's Industree Group Stadium was pushed back from Sunday after a waterlogged pitch made the surface unplayable.
Central Coast had more than 60 per cent possession in each half, but Canberra looked more threatening on the counter attack In the first 45 minutes.
Matildas stalwart Heyman opened the scoring in the 43rd minute, arrowing a fine shot into the net after out-muscling Mariners captain Taren King, who was playing her first match of the season after a knee reconstruction.
Heyman, who turns 37 in July, boosted her team-leading goal tally for the season to eight and became the oldest player to score in an ALW finals game.
It took the Mariners just 22 seconds of the second half to equalise
A Tiana Fuller cross to the far post was headed across goal by Annabel Martin for the centrally positioned Pennock to fire home.
"Good on them to come out firing. We knew they were going to, but we were just hoping it wasn't that quick," Heyman told Paramount Plus and 10 Bold.
Canberra were not as dangerous after halftime and Pennock settled the game with a sumptuous 74th minute strike, rifling a right foot shot from the edge of the penalty area past United goalkeeper Sally James.
Pennock had scored just five goals from 71 shots before Monday's game.
"I was disappointed in the last game. I had a few chances that I think I should have scored and I didn't put away, so I've worked hard all week in training," Pennock said.
The Mariners have made the semi-finals in each of their first two season back In the competition.
They will play unbeaten minor premiers Melbourne City at home on Saturday In the first leg of their semi.
One big watch for the Mariners is Taylor Ray, who hobbled off with a knee injury shortly before the end.
