The former Middlesbrough winger signed a four-year contract after the two clubs agreed an undisclosed fee.
He told Sunderland's website: "It's great to have signed and it's a fresh start."
He went on: "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."
England international Johnson missed out on a trip to Euro 2012 and will hope his latest move could help to revive his international career.
Johnson played 96 times for first club Boro, scoring 13 goals, before he moved on to City in 2010.
But after 73 games, which produced 11 goals, he has now moved to Sunderland.
He said: "I want to perform on the pitch for the fans and I'm just looking forward to getting out there now."
Source: PA
Source: PA