The winger scored the only goal of the game against his former club as the Black Cats gave everything they had to shut out the champions.
"You could see what it meant to everyone at the end there," said Johnson to safc.com.
"It was a fantastic three points against a team who are the champions.
"There was a fight in us. We worked so hard as a team and got our rewards for that work.
"City were always going to create chances but we had some too."
Johnson added: "I tried to mute my celebrations a little because I had great times at City. Any club where you win medals and progress as a player, you have to show respect."