The 26-year-old's form in the second half of last season was a major factor behind the Black Cats avoiding relegation, and saw him awarded the Premier League player of the month prize for January.
Johnson has 12 caps to his name but last appeared for England in August 2012 after struggling impress Roy Hodgson - and he feels his performances since the turn of the year warranted a place on the plane to Brazil.
"On the whole, since the new gaffer [Gus Poyet] came in, I've been happy with my form," he told the Sunderland Echo.
"It was then that I started scoring a few goals and getting back to my best form and from January onwards is probably the best I have played in my career.
"Everyone would be saying that I was playing really well at City when I was getting into England squads, but personally I think this is the best spell I've had as a footballer.
"I think it's the freedom I've been given; expressing myself, playing my normal game and not having to worry about playing right-back, which brought the best out in me.
"Or not getting the ball on halfway and having to run 60 yards. It's all those little things. You ask any attacker and the last thing you want to be doing is running 60 yards before you get anywhere near the goal."