Johnson opened the scoring in the 39th minute after he capitalised on a horrendous error from Wigan defender Titus Bramble.
Phil Jagielka's own goal gave Wigan hope but the visitors held on for a win which takes them fourth in the Barclays Premier League.
"Wigan are a good team and this is a hard place to come and get a result," Johnson told Sky Sports 1.
"Physically this is one of the hardest games we'll play but the important thing is that we got the three points and we're sitting pretty in the league.
"For the goal I just chased it down and when the chance came I managed to put it away."