The 25-year-old penned a four-year deal at the Stadium of Light after being made aware by City boss Roberto Mancini that first-team opportunities would be limited for him this season.
Johnson started his career at Middlesborough and spoke of his delight at return to familiar surroundings as he tries to reignite the form that has seen him reach the fringes of the England setup in recent times.
"It feels fantastic [to be here]," he told the club's official website. "I was born here and there have been rumours in the past about me coming here.
|MACKEMS RUN RIOT
|20/1||Sunderland are 20/1 with William Hill to beat Morecambe 5-0 in the League Cup on Tuesday|
"It's great to have signed and it's a fresh start.
"Coming back to the North East and Martin O'Neill were the two main factors, especially the gaffer and listening to what he had to say to me.
"He made it clear just how much he wanted me to come here. To know that and feel wanted and appreciated made me feel that it was the right choice to sign.
"He knows exactly what he wants from his players and from me personally. Hopefully he'll give me plenty of confidence as well."
Johnson had been linked with moves to such teams Liverpool and Tottenham before joining Sunderland.
And O’Neill believes the capture is a “real coup” for the club, in light of the reported heavy competition the Black Cats faced to land the winger.
"To get Adam, it's really, really big news for us, big news. It's a real coup,” O’Neill added.
"He's a fantastic footballer and he's really exciting. I just think if he stays clear of serious injury, he will be great for us, really great.
"I am pleased he has chosen to come here because at Manchester City, he might not have been playing all the time there and maybe it seemed as if he might be leaving.
"But to come to us instead of a number of other teams who were definitely in for him, I was just very, very pleased, pleased for everyone."