After spending the majority of the season inside the top six, Paul Jewell's side must now win their final game of the season against West Ham to stand any chance of reaching the Barclaycard Premiership.
Johnson needed a huge slice of fortune to register his fourth goal in three games since returning to the starting line-up following a six-month injury lay-off.
After linking up well with Andrew Impey, Eoin Jess crossed to the feet of Johnson, who slipped while attempting to shoot.
From the resulting scramble Paul Evans looked certain to score his first goal for Forest, but his goalbound shot struck Johnson before crossing the line.
Wigan rallied after Andy Liddell replaced Alan Mahon but were unable to grab an equaliser and have now taken just two points from three games.
Jason Roberts' suspension meant Lee McCulloch started alongside Nathan Ellington in the Latics' attack and the striker did not wait long to remind Forest he is no less dangerous without his partner alongside him.
After Michael Dawson had cleared Gareth Farrelly's corner Ellington collected the rebound to test Paul Gerrard with a smart shot.
Joe Kinnear made just one change to the side which was so impressive in defeating Ipswich last weekend. Out went the suspended Wes Morgan to be replaced by Des Walker, fit again after a knee injury.
Forest, who have also beaten Wigan's play-off rivals Sheffield United and Crystal Palace since Kinnear took charge, rarely threatened John Filan's goal with Premiership-bound midfielder Andy Reid their main creative influence.
The Republic of Ireland international, widely understood to be a transfer target for Tottenham and Charlton, created the hosts' best chance of the half when his inswinging corner found the head of Dawson. The defender made solid contact but his header flashed wide of the upright.
Filan was forced into action when Marlon King chanced his arm from distance, but the Wigan keeper was able to get a firm hand to the striker's rising drive.
An ankle injury forced Gerrard out of the action shortly after the break. His replacement Barry Roche, who endured a torrid time when thrust into the spotlight earlier in the season, was beaten by Ellington shortly after the hour but the striker saw his shot come back off the post. Tweet

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