Johnson, who completed a move to the Stadium of Light last week for £10m, admits that he can already see a difference in mentality from Roberto Mancini's regime at Manchester City.
He said: "Leaving City, leaving the champions, was one of the biggest decisions of my life, but hopefully coming here will make me a better player.
"Martin O'Neill doesn't want us to just be a mid-table team and I want to play week in, week out. Coming here, I know I can enjoy playing football again.
"The other night when I made my debut (at home to Morecambe in Tuesday's Capital One Cup win) O'Neill gave me confidence and freedom, it was as if I'd had the chains taken off me."
Johnson is in the squad to make his Premier League bow for the Mackems this weekend as they travel to Swansea City.