In one of the most brilliant finishes to a John I Dent Cup grand final in recent memory, Johnson cut a swathe through the Vikings defence to secure the 21-17 victory against the Tuggeranong Vikings at Viking Park on Saturday.

Royals supporters flooded the ground in scenes of unbridled jubilation as they celebrated the club's first premier division title since 1991.

"I remember throwing the ball up in the air after the try, I don't remember scoring it," Johnston said.

"It was insane, best feeling ever.

"I'll have to watch it back again."

The Vikings appeared to have secured their fifth straight premiership when Wallabies representative Christian Lealiifano slotted a 77th-minute penalty goal to give his side a one-point lead.

Royals snapped a run of six losing grand finals, coming from fourth place on the table to pull off the impossible.

It was the perfect way for Johnston to cap off his 100th game in his 11th season for Royals.

"When I arrived we were doing it pretty tough, but we've been slowly building," Johnston said.

"To be part of this club is like nothing I've experienced.

"It's like one big family, they welcomed me with open arms when I moved down here and I can't thank them enough."

Veteran Royals prop Jay Cash - who scored in the 70th minute to put his side ahead 16-14 - declared he would be going out on a high after 16 seasons at the club.

"It means the world," Cash said.

"Not just to the players, but to everyone involved with the club, it's been 24 years.

"[When Johnston scored] I was on the ground having a sleep, I'm too old for this.

"We had the belief and we knew we had the team."

Royals coach Wayne Southwell said he was delighted to be a part of a wonderful team performance given the tough road his club has endured.

"That's a real fairytale," Southwell said.

"To come from fourth, win it right on the bell and a game that had everything.

"I'm really happy we hung in there at times when we didn't play so well, but belief is everything."

Vikings coach Brad Harris praised the resilience of Royals for staying in the contest.

"We got ahead at the end, but we didn't close it out," Harris said.

"Full credit to Royals, they never went away and got the result.

"We knew that would be the case with the number of quality players they have."

It was a tough day all around for Vikings, who lost all six grand finals.

A penalty goal from Royals five-eighth Craig McMahon were the only points of a defence-minded first half.

Lealiifano put the Vikings in front in the 42nd minute when he ran a perfect line to score the game's opening try.

Another penalty goal from McMahon in the 47th minute brought the Royals back to within a point before Vikings flanker Rowan Perry was sin-binned in the 52nd minute for pulling down in the line-out.

Royals made the most of the extra man, a brilliant kick from Brumbies winger Lausii Taliauli setting up a try for outside centre Jason Swain.

Vikings hit back in the 61st minute when Jerome Niumata was open on the right wing, Lealiifano converting from the sideline to put the Vikings ahead 14-11.

There was also a thrilling finish in the second-grade decider, the Queanbeyan Whites scoring a try after the siren to snatch a 29-28 victory.

JOHN I DENT CUP GRAND FINALS

Premier Division: Canberra Royals 21 (Jason Swain, Jay Cash, Ben Johnston tries, Craig McMahon 2 penalty goals) bt Tuggeranong Vikings 17 (Christian Lealiifano, Jerome Niumata tries, Lealiifano 2 conversions, penalty goal) bt at Viking Park on Saturday. Second grade: Queanbeyan Whites 29 bt Tuggeranong Vikings 28. Third grade: Queanbeyan 25 bt Tuggeranong Vikings 20. Fourth grade: Wests 31 bt Tuggeranong Vikings 29. Colts: Canberra Royals 20 bt Tuggeranong Vikings 17. Women's: Canberra Royals 27 bt Tuggeranong Vikings 22.