Johnson surprised by England World Cup snub
Johnson's form in the second half of last season was a major factor behind Sunderland managing to avoid relegation, with the 26-year-old even being awarded the Premier League player of the month prize for January.
The former Manchester City man has 12 international caps to his name, but has not played for his country since August 2012.
Johnson was a regular part of England squads under previous manager Fabio Capello, but has failed to force his way into the national team set-up under the Italian's successor Roy Hodgson, with the likes of Adam Lallana and Raheem Sterling jumping ahead of him in the pecking order.
However, Johnson feels his performances after the turn of the year were good enough to warrant 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 Shields Gazette.
"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."