The 25-year-old completed a £10m move to the Black Cats last Friday, with the England international penning a four-year deal at the Stadium of Light.
Johnson, who scored 15 goals in 97 appearances for City, revealed the promise of first-team football was a key factor in his decision to quit the Eastlands outfit.
“When you’ve been at a club like City, the champions, with all those world-class players, it’s a difficult decision in terms of deciding you have to go,” Johnson told the Shield Gazette.
“And then after that, it’s a difficult decision deciding where you’re going to go to. I had a lot of thinking to do.
“I just felt I had to leave City for myself, because I wanted to go somewhere where I could play week in and week out, and enjoy myself.
“There were a lot of clubs in for me, but the manager was a big influence on me, I could also get to see my family.
“But quite honestly, there was a lot of things that swayed my decision, a lot of things behind the scenes that came into it.”
Johnson also added Sunderland boss Martin O’Neill played an important part in selling the club to the former Middlesbrough winger.
“The manager has been brilliant. He’s the sort of person who fills you with confidence, and I respond to that,” Johnson added.
“I also like his ambition too, and what he wants to do with Sunderland.
“He’s not just content to be a mid-table manager, he wants to take the club higher, and I want to be part of that.
“I’m from around here, so I know it’s a case of swapping one massive club for another.”
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