City had been taking a pounding and were just about holding on when a lunging clearance from a late desperate counter-attack fell to him. All of 25 yards out, he hit it back past startled keeper Matt Glennon who dived late as it landed just inside his left-hand post.
Minutes before, Johnson - son of manager Gary Johnson - had been caught up in a touchline row with a fan calling for him to be taken off.
After scoring, he wheeled away from the congratulations of his team-mates to cup a hand to an ear inviting the applause of the crowd.
Their cheers were of relief as Johnson knows he has yet to get them on his side after being signed from Hearts at the start of the season.
Father, Gary, brushed aside the row with the fan. "It was just one idiot who has something on his brain," he claimed. "You always get someone who wants to pick on a player and in the dug out you hear some horrible comments.
"I always pick players on merit and we know the stats of what every player contributes."
Until Johnson pulled out his get-out-of-jail strike, City were heading for their third league defeat in ten days.
There was a lack of creative play in the final third and they had failed to deal with the cutting edge provided by Michael Collins on the right and Danny Schofield on the other flank.
Town boss Peter Jackson took his side down to the West Country a day early, not only to beat the weather, but also to give his new players Frank Sinclair and Andy Taylor time to bed in.
The strategy worked as, apart from an early chance for home striker Enoch Showunmi well turned over the bar by Glennon, it was Huddersfield who took over the flow of the game.
Their goal when it came early in the second half was a gem. Gary Taylor-Fletcher advanced down the middle, dribbling past three defenders without a tackle. From around the penalty spot he tucked his shot almost gently into the bottom right-hand corner.
Two minutes later, home keeper Adriano Basso did well to finger-tip a shot from Collins around the post.
It was almost his last contribution. Minutes later he ran outside his area to stretch for a chip from Luke Beckett, who had run on to a ball from Andy Booth and was immediately red-carded.
Stand-in keeper Chris Weale, playing his first game of the season, did well but City know they could easily have finished well-beaten if Town's finishing had matched their enterprise in approach.