It capped a remarkable turnaround for the striker, whose attitude as much as his finishing ability is in the spotlight this season.
He was heading out of The City Ground exit door at the end of last year when he was transfer-listed by manager Gary Megson as part of the Forest clear-out.
But Johnson has fought his way back into the picture and got his reward with the 87th minute clincher.
Johnson admitted: "I've got a lot to prove to myself and other people this season, and hopefully this is just the start.
"I know I didn;t do myself justice last season, and I've worked hard during the summer to improve things.
"The manager has given me a clean sheet to show what I can do and it's an opportunity I want to make the most of during the season."