The Millers secured their third promotion in as many League One seasons under Warne's management with a 2-0 win at Gillingham but it was hard work.
Rarmani Edmonds-Green gave them a first-half lead but a Gills side fighting to saty in League One refused to give up, hitting the crossbar through Ben Thompson and keeping the result in doubt until Football League debutant Kelly came off the bench to score an 89th-minute winner.
"Smudge (Michael Smith) has been playing with a broken foot for weeks, getting jabbed up all the time, bless him," said Warne.
"He's struggled with his quad but he's that good even on one leg we try to keep him on because he causes defenders problems and helps us defend our set pieces. He's fundamental to this club's success.
"But it just felt a little bit like Smudge couldn't move and it was a bit Roy of the Rovers. I could have put Kayode on, he's been great for us as well, but I just thought the way we were crossing the ball Georgie's really aggressive in the six-yard box - I say that and he scored with his feet.
"It was just one of them Roy Race moments and he'll be remembered forever. He's been interviewed by Sky and he's only played seven minutes in his Rotherham career but he's up there with Alan Lee status.
"He's trained really well, deserved his chance and he took it, so fair play to him."
Rotherham have been automatic promotion contenders all season but a late wobble in the final weeks put their elevation in doubt.
As with everyone else in the table after 46 games, though, they finished where they deserved to.
"We've had a good season and I don't think many people will complain that we've gone up although in fairness to MK (Dons, who finished third), they've ruined the last six week so of my life doing really well to get into the position they have and I feel sorry for them just coming up short," said Warne.
"It's a pity we didn't top it off with the title. If we'd hung on in midweek we possibly would have but it was not to be. I'm not complaining, I'm really proud of the lads.
"Some clubs after relegation go down another league but we signed really good people who have the right intentions in life and 10 months' hard work has paid off so it's a nice feeling.
"My honest emotion is relief. We've been in a really good position for weeks and sometimes you feel the soccer gods are against you but the lads have really rallied and finished the season off well.
"It sums it up that Smudge has been playing with unbelievable pain in his foot and his quad. He had to come off, Georgie Kelly came on who's been training and looking after himself as well as he can and he scores the winner. I think it just sums up the resilience of our players and their hunger for success. They're not big-time. Will Grigg's here and Shane Ferguson managed to run with a boot on.
"To get automatic promotion and a cup win is something this club has never done before and it will always go down in folklore. I'm really proud to be a part of it.
"I think we could have been three up at half-time although they could have scored when they hit the bar, which we weren't best pleased with.
"We knew what their threats were and Neil (Harris) has really organised them. They were well out of the league and the fact they got to this game with the chance to stay up was credit to him and he was a brilliant gentleman on the side of the pitch.
"But we just couldn't get the second goal and while you haven't got the second goal there's always that chance, there's always that long throw into the box or whatever.
"Although they didn't really trouble our goalkeeper there's always that chance."
"The second goal was crucial and it was Roy of the Rovers stuff that Georgie got it."
The game ended with supporters on either side trading punches on the pitch, delaying Rotherham's celebration there until 3.30pm, an hour after full-time.
"Nothing's going to taint the lads' enjoyment today - we've got to the Championship and we know the size of our club," said Warne from a gangway in the Priestfield Stadium waiting for the go-ahead to go back out. "We've played really good football at times and I thought we thoroughly deserved our win and the lads deserve all the praise we can give them."