Johnson, who moved back to his native north-east for pounds 10?million in the summer, would probably not have wanted to expose Hart's poor decision making, but he will have -thoroughly enjoyed causing discomfort for the Italian, who never took to him as player during their 21/2 years together in Manchester.
Mancini dismissed suggestions that Hart was at fault for the goal, preferring instead to divert attention by criticising the referee, Kevin Friend, for eating "too much for Christmas", but the England goalkeeper was to blame and it was City's players who looked bloated and lethargic, not the match officials.
Hart made a fatal error for a goalkeeper - he assumed Johnson was going to cross the ball, moved off his line, allowing the winger to squeeze a shot from 25 yards inside the near post he had abandoned. Mancini may have had a point about Pablo Zabaleta being fouled by Craig Gardner in the build-up, but it was a tight call rather than a blatant one and Sunderland still should not have scored.
Hart's shot-stopping generally remains as good as ever. Indeed, he made two excellent saves to deny Steven Fletcher in the first half and Stephane Sessegnon in the second, but his positioning and decision making is starting to become a concern for both club and country.
Criticised for letting Robin van Persie's free-kick creep past him in the Manchester derby earlier this month, it is not so long ago Hart was also being blamed for rashly coming off his line to try and claim a cross that allowed Poland to equalise during England's World Cup qualifier in Warsaw.
That, though, should not detract from a fine Sunderland performance. Neither should Mancini's assertion they had been "very lucky" to beat them. They were not. City had chances and may well have been "too soft in attack" as their manager claimed, but they were also bullied at times by the Black Cats, who also had opportunities to add to Johnson's strike.
Sunderland's win was down to more than luck, particularly when you consider they have now taken seven points off City in the space of 12 months. "They are a bit of a bogey team for us here," conceded Mancini, whose side also drew with the Black Cats at the Etihad Stadium last season. "I thought we did enough to win the game. Nothing went wrong for us. We had lots of chances and played well, but we did not score and this is our problem. Whenever we have chances, we are too soft. We are too soft in attack, too soft when we get chances, too soft when we shoot. Sometimes we don't shoot. We want one touch more and that is no good."
This was Sunderland's third successive home victory over the reigning champions and their second under Martin O'Neill, following Ji-Dong Wong's last-minute winner on New Year's Day. This, however, was a far better all-round performance.
Yes, Sunderland defended for their lives at times and goalkeeper Simon Mignolet - linked with a move to Arsenal last week - was magnificent, making three superb saves to deny Yaya Toure and David Silva in the first half and Sergio Aguero in the second, but this was an O'Neill side playing as an O'Neill side should.
Sunderland were spiky, brave and played with width, where wingers Johnson and James McClean were constant trouble for City's full-backs.
Sunderland survived a nervous opening 20 minutes in which Vincent Kompany headed against the bar before Mignolet blocked Toure's follow-up with his feet and gradually got to grips with the visitors in midfield and nullified their attack.
"Considering the opposition this was our best performance of the season," said O'Neill. "We knew we would have to defend strongly and do well without the ball, but we also carved out chances.
"We played really well. Not just defensively. We moved the ball well and caused them problems. I'm really not bothered what Roberto said about it. He said we were lucky when we beat them in January and drew with them at the Etihad. I've come across it before with other managers when they lose. I thought we were terrific."
The defeat leaves City seven points behind Manchester United at the top of the table, although Mancini stuck to his "it's still early in the season" mantra. "This doesn't change anything," he added. "At this moment, United are better than us, they are at the top, but we have time to recover. It is no problem seven points."