The former striker became a cult hero and scored one of Newcastle's most important goals before playing for their great rivals.
Crossing the Tyne-Wear divide never usually ends well; ask Jack Colback or Lee Clark, but Kelly was an exception.
The striker moved to St James' Park from Leicester in 1991 and departed in 1993, having played 70 games and scoring 35 goals.
However, none were as crucial as the one in 1992 against Portsmouth.
The Magpies faced relegation to the Third Division with just two games of the season remaining.
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Kelly scored the winner in a 1-0 victory, saving the club from demotion.
Speaking in Newcastle United Match of My Life, by talkSPORT's Alex Crook and co-author Jake Rusby, the forward recalls the game.
He said: "In terms of my winning goal, for several years afterwards, I thought it was from outside the penalty area and didn't realise it was five yards inside the box until I watched it on YouTube.
"It was still a decent strike. It also came at a good time of the game, with only five minutes remaining.
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"That was our penultimate game of the season, and we then travelled to my old club, Leicester, on the last day to win 2-1 at Filbert Street. It didn't make any difference because we ended up winning, and, in fact, that victory over Portsmouth would have kept us up anyway."
Had Newcastle been relegated that season, who knows where they would be today?
But that goal changed everything, and the Toon stormed to promotion under manager Kevin Keegan the following year, with Kelly leading the line with Andy Cole.
Since then, the Magpies have played in the Champions League and competed for the Premier League title, but their fans will never forget that Kelly strike and immortalised it with a flag.
The tribute by Wor Flags was unveiled during a home game against Leicester in 2022, and Kelly was blown away.
He said: "Seeing the tribute Newcastle fans paid to me before the Leicester game was a truly humbling moment, especially because I didn't know anything about it.
"I was quite emotional to be remembered for something that happened 30 years in the past, and it obviously stuck in the mind of a lot of my generation. It was incredibly special."
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Unfortunately, the former Republic of Ireland international never got to represent the Toon in the Premier League because Keegan moved him on.
However, the striker isn't bitter about it and recalls his conversation with his manager in Newcastle United Match of My Life.
He said: "Once the season had finished, I was driving back down the motorway to Birmingham when Kevin phoned and said we needed to have a chat, so I pulled into a service station and said, 'Gaffer, are you selling me?'
"He told me offers had come in from Wolves and Nottingham Forest - and the club had accepted Wolves' offer.
"He asked if I'd be happy to talk to their manager, Graham Turner, and I agreed, but only on the proviso that Kevin told me who he was bringing in.
"He refused, as it was a big secret, so I said, 'f*** it, I ain't going then'.
"Kevin phoned me back about ten minutes later and said we needed to sort things out, and it was then he reluctantly told me they were bringing in Peter Beardsley. My exact words were, 'F****** hell gaffer, I'd sign Peter Beardsley instead of David Kelly!'"
With that, he was off and joined Wolves before returning to the north east and signing for Sunderland in 1995.
Kelly wasn't as successful on Wearside, making 34 appearances and scoring two goals, but he was treated to a special reception at St James' Park.
Recalling the moment, he said: "When I came on as a substitute in a Tyne-Wear derby at St James' Park in April 1997, I received a standing ovation from the Toon Army, even though I was wearing red and white. That was definitely unexpected.
"It was very strange but much appreciated.
"I scored the winner in my first derby game against Sunderland at St James' Park, so I think that helped my relationship with the fans."
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Moments make players, and that goal against Portsmouth means that Kelly will always be in Newcastle fans' hearts.