The 25-year-old winger thought 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 of the net.
Adam Johnson:"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.
"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.
"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.
"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."