This was the first time Forest have played in the third tier of English football in 54 years and visiting Huddersfield gave them a difficult welcome.
And it seemed as though Forest would pay the price for a vast number of missed chances when a 64th minute Pawel Abbott penalty cancelled out Scott Dobie's opening goal on the stroke of half time.
But Johnson, who had been frozen out and transfer listed by manager Megson following a bust-up last season, came off the bench to spare Forest's blushes as he got most of the 24,042 crowd on their feet with an 87th minute winner.
The first half had seen Forest carve out the best of the chances in what was a scrappy affair which fitted all the stereotypes of lower division football.
But it was not until injury time that they were able to make the most of their numerous opportunities when Dobie intervened.
Latching onto a superbly flighted cross from Kris Commons, the frontman neatly chested the ball down before hitting a looping shot that keeper Paul Rachubka could only palm onto his crossbar, leaving Dobie to head home the rebound from close range.
Dobie had initially been one of several players to spurn opportunities as he led a quick break from the Forest half in the first ten minutes.
However, with the home side outnumbered, Dobie's attempted through ball to Commons was cut out by Martin McIntosh.
Huddersfield almost snatched the lead as Danny Cullip's attempted headed back pass fell short forcing keeper Rune Pedersen to make a desperate block at the feet of Abbott.
Otherwise it was Forest who largely dominated but Gareth Taylor, Dobie and Commons all narrowly missed the target with headers while Gary Holt hit a 20-yard drive that was also just off target.
In the second half the story was the same with Forest continuing to carve out and miss their chances.
Commons connected superbly with a volley but directed it wide from 12 yards while lone striker Taylor continued to find good positions but could not find the killer touch as he forced just one save from Rachubka from a trio of headers.
And Forest paid the price as Huddersfield claimed an equaliser against the run of play.
Substitute Danny Schofield had been on the pitch for less than four minutes when his twisting run into the box was brought to an abrupt end by Cullip.
Referee Nigel Miller pointed to the spot and Abbott calmly converted sending a lot shot to Pedersen's right.
The goal lifted the visitors who suddenly looked to be a threat and they could have snatched the lead as David Mirfin picked out Andy Booth with a fine cross for the veteran to connect with a thundering header that Pedersen saved brilliantly.
But fans' favourite Johnson came off the bench in the 74th minute and, after being sent clear by Commons, beat Rachubka at the second attempt after the keeper had initially saved his driven shot.