McGrath lived out his boyhood dream by calmly nailing the winner from outside the 50m arc after the final siren.
It capped a remarkable Lions comeback that saw them overturn a 52-point deficit late in the third quarter to win by five points.
To add to the fairytale script, it was McGrath's 200th game.
"It's a boyhood dream. Everyone does it when they're a small kid, kicking into goals or kicking through a gate," McGrath said.
"You always dream of kicking the winning goal to get your team over the line. It's just a surreal feeling at the moment.
"I was lucky it went through. The ball didn't come off that well.
"I thought it was going to hit the post. Full moon, I think my old man up there must have pushed it through."
The Lions trailed by a point inside the last 30 seconds when full-back Daniel Merrett took a mark in their defensive goal square.
The ball was quickly swept via a chain of possessions that included Simon Black, Jed Adcock and ultimately Dayne Zorko, who passed to McGrath.
There was almost no time left when he took the mark.
McGrath said he only faintly heard the siren over the cheers of 24,164 raucous fans.
Although his teammates didn't offer much advice as he lined, he said there was plenty coming from his Geelong opponents.
"You can take as much time as you want, but I thought I'd just go through the process and don't hang around too long and think about it," McGrath said.
"The crowd was as loud as I've heard it at the Gabba over my 13 years.
"It's a boyhood dream. You do it a lot when you're growing up out in the paddock. I finally got a chance, it's taken 200 games … I'll hand it over to someone else next time."
Michael Whiting covers Brisbane Lions news for AFL.com.au. Follow him on Twitter: @AFL_mikewhiting
