King's fellow defender broke the hearts of City fans in the 10^th minute of extra time by charging onto the end of a Luke Brattan cross and chesting the ball into the back of the net.

"We were battling quite bit to get the goal and it took us longer than 90 minutes. I knew it had to take something special for us to break through the City defence," Shellharbour teen King told the Mercury.

"But it didn't surprise me that Luke Brattan put one straight onto Rhyan Grant's chest and he put it away into the bottom left.

"I've been watching Rhyan Grant for a while so I know his qualities and how good of a player he is. He is a Socceroo and he deserves to be. For him to pop up and score a goal didn't surprise me one bit."

Grant's goal proved the difference in an enthralling decider at Bankwest Stadium, as Sydney FC clinched their fifth A-League premiership. They also claimed this season's title double, after winning the Premier's Plate.

However, it could have been a much different scenario. King was at the centre of a controversial decision in the first half when the VAR overturned a goal by Melbourne's Harrison Delbridge.

The VAR ruled that an offside Lachlan Wales had obscured the vision of FC goalkeeper Andrew Redmayne.

"The ball went straight through my legs so I wasn't too happy about it. But, then walking back to halfway, I saw that VAR was checking it," King, 19, said.

"Looking at it now, I'm definitely not complaining. I think Lachy Wales was in the way of 'Redders' and he was off side, so I think it shouldn't have been a goal.

"I think in those first 15 minutes, we were just under the pump and then when it [goal] was disallowed, it gave us a boost of energy. From then, we were switched it on. We turned it on and played some good football to fortunately get the win."

But that moment of agony would change to ecstasy by the full-time whistle after 120 minutes. After an unprecedented A-League season, which had a stop-start nature due to the COVID-19 pandemic, King said it was a surreal but great feeling.

"When that final whistle went, I didn't know what to do. I was a bit lost, I just screamed and went to 'Wilks' [Alex Wilkinson] and gave him a big hug," he said.

"I tried to soak everything in because I know winning a championship doesn't come too often. I was just grateful and really happy."

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While King was celebrating with teammates, former Sydney FC player Jacob Timpano was reminiscing in his Illawarra home. Timpano, who won the A-League title in 2006, said it was a proud moment for King.

"It's a fantastic achievement. He's only 19, he's got his whole career ahead of him. I really like Joel King as a player, he gets up and back and covers ground on that left side well and really adds something to Sydney FC," Timpano said.

"It certainly brought back some memories of winning the first A-League title. I was only young and to think Steve Corica scored the goal [1-0 v the Mariners] and he's now the Sydney FC coach.

"It's pretty special to think they've now won three of the last four grand finals and become the most successful club in the A-League."