The former Middlesbrough winger, raised in a family of Sunderland supporters, silenced the Stadium of Light crowd with a stunning left foot strike that underlined why Bruce had been so keen to land the Easington-born star.
His goal in the second minute of added time cancelled out Kenwyne Jones' first half header that looked set to give Bruce's revitalised side a deserved win.
Sunderland missed Jones' towering presence after he was withdrawn at half-time and Bruce is concerned the Trinidadian could be sidelined for a month after revealing the striker suffered a groin strain, while concerns about Lee Cattermole's fitness persist.
The heroics of Sunderland goalkeeper Craig Gordon looked to have grabbed Bruce back-to-back wins for the first time since he took charge last June until a moment of sublime skill from Johnson handed City a share of the points.
For Sunderland it was a cruel time for Johnson to open his account with Roberto Mancini's side but Bruce and owner Ellis Short, who again watched from the directors' box, will have been heartened by another spirited home performance.
The Wearsiders are unbeaten at in their last six home league games and their position seven points clear of the relegation spots has cooled talk of relegation.
A rueful Bruce reflected: It's ironic it would be Johnson, a Sunderland fan. I can see the headlines. I was on his case for months, unfortunately he chose Man City which was disappointing for us, but there you have it.
He's something different because he's a dribbler, he wants to create things and get at full-backs.
He is a natural footballer and a very good player.
Despite City boasting talent such as Johnson, Roque Santa Cruz and Patrick Vieira on their bench it was Sunderland who played all of the football in the first half and they should have added to Jones' eighth goal of the campaign.
Cattermole was rested after failing to recover from hamstring trouble and Bruce noted the midfielder was nowhere near to playing yesterday.
Lorik Cana served the first of his two game suspension but replacements David Meyler and Kieran Richardson dominated Nigel De Jong and Gareth Barry in the visitors' midfield engine room.
We were excellent in the first-half; the best we've played for weeks, said the manager.
Them bringing on Santa Cruz after 30 minutes tells you everything. The loss of Kenwyne was defining for us because over the last month he's played ever so well.
It was possibly a fair result, but the way it came about is hard to take. It looks like Kenwyne will be out for a month with a groin problem.
We've had eight who've been out for more than eight weeks in a squad of 22 and that pushes us to our limits.'' Bruce was quick to hail the efforts of his goalkeeper to keep City at bay.
Craig's a very good goalkeeper and I thought he'd done enough to win us the match,'' he said.
The search for back-toback Premier League wins goes on.
We were ten seconds from keeping our fourth straight clean sheet at home but it wasn't to be.
It's deeply frustrating and we have to think what could have been. We could quite easily have been sitting here on 41 points.
Neverthless Bruce was looking towards the safety of mid-table rather than eyeing the Premier League relegation zone.
It's our aim to get into that top ten and we'll try to finish off strongly.
With a settled team we'll be okay because we're showing a good resilience.
Man of the match Gordon, who pulled off a succession of superb saves to deny City's second half onslaught, was devastated to concede another late strike.
We are defending well but we have conceded a few goals late on and that has been disappointing.
It was an unbelievable finish right in the top orner and no-one was going to stop that.
But I think we have turned a corner.
Asked if Johnson was worthy of an England World Cup berth, Mancini said: I think that Fabio Capello knows all the players.
Adam's a good player but he's a young player.
He'll be a good player for the national team but maybe not just now.
Maybe this World Cup will come too soon for him but it's not me who decides.
England's national team has a good manager who knows Adam well and he will decide.
Mancini said: He scored a fantastic goal. I am happy for him and for us, but before that, we had five or six chances easily to score a goal. That's football.
The Sunderland goalkeeper was fantastic today. He saved everything.''