The 25-year-old winger stood with his hands on his head on the pitch at the Stadium of Light on Wednesday after his England international team-mate had hurled himself across goal in an attempt to keep out his dipping left-foot strike.
Hart got to the ball after initially appearing to be wrong-footed, but several seconds elapsed before it came clear that he had been unable to prevent it from creeping under him and into the bottom corner to spark belated celebrations by Johnson and his team-mates.
The Sunderland man said: "I know Harty is probably the best keeper in the world and I have seen him many a time save those.
"I thought he had saved it and it had gone under him and around the side of the post, not in the net. I couldn't see the ball because he was covering it.
"But obviously when the crowd cheered, I knew it was in and that's when I realised. It all happened so quickly."
Johnson scored his first City goal against Sunderland on Wearside in March 2010 to snatch a 1-1 draw after coming on as a substitute, and he admitted there might have been an element of fate about his latest contribution to the fixture.
He said: "It's crazy, isn't it, how football works? I scored the equaliser for City and to come back now and score the winner for Sunderland against City, it's just how it works, football.
"It was in the script, I suppose."
Johnson's decisive intervention came eight minutes into the second half of a game in which Sunderland at times had to defend for dear life with keeper Simon Mignolet turning in yet another superb individual display.
He needed the help of the crossbar to keep out Vincent Kompany's early header, but he denied Yaya Toure from the follow-up and later David Silva, and after Johnson had struck, went one-on-one with Sergio Aguero and again came out on top.
The visitors may not have been at their clinical best - manager Roberto Mancini later accused them of being "too soft" in front of goal - but the Black Cats gave as good as they got for long periods and had it not been for Hart, who saved well from Steven Fletcher before the break and misses by Stephane Sessegnon and James McClean after it, they might have won more comfortably.
Johnson said: "Whoever they [City] play, they are always going to get chances and they are going to create things with the players they have.
"But we stood strong and we battled hard and Simon made a couple of great saves.
"But we gave them as good as they gave us. We created a lot of chances and Harty made a couple of good saves for them.
"Overall, we definitely deserved something and we will take the three points.
"It just shows the quality of the players in the squad that they are not losing games very often, and we are just delighted to beat the champions. It's a massive win for us."
Individually, Johnson turned arguably his best display yet in a red and white shirt following his £10million summer switch, combining his attacking duties with helping out at the back.
He said: "When you play for Sunderland, obviously there's a bit more onus on you to do a bit more defensive work. When you are at City, you have the ball a lot more and you can afford to play higher and not do as much defensive work.
"But I am bringing that back into my game from my Middlesbrough days. I have got to defend a lot more and I am definitely getting better and fitter at that.
"The main thing I want to do is attack and score goals, but I will never forget my defensive duties."