At the time the hosts were facing a rearguard action against the struggling Saints when goalkeeper Adriano Basso rose to catch a late corner.
His opposite number Kelvin Davis had gone forward for the corner and when Basso's throw fed Sproule on the left, the flying winger ran fully 70 yards past a covering defender before slotting into an empty net.
Johnson said: "Ivan is the quickest player I've worked with and as soon as he got the ball I knew a goal was on.
"The only question was whether Ivan would find the net or run straight down the tunnel behind the goal! Thankfully he got the radar right in the end.
"Southampton put us under pressure in the second half and all credit to them for that. But we defended when we had to and earned our win.
"Things are going very well for us and we are confident about making the play-offs, not in an arrogant way, but because we have faith in one another."
Dele Adebola had put City ahead when he beat the Saints' offside trap to run on to Bradley Orr's ball over the top on 34 minutes and confidently shoot past Davis.
Orr had already missed a 15th-minute penalty when he shot too close to the Saints keeper after Rudi Skacel had fouled Nicky Maynard as the striker attempted an overhead kick.
But Saints had their moments in the first half and both Morgan Schneiderlin and Jason Euell shot against the post with Basso beaten.
Euell missed a great second-half chance to level the scores and Saints coach Mark Wotte was left hugely frustrated.
"I cannot believe we lost the game," he said. "The least we should be going home with is a point.
"I can't fault the players for what they did in the second half. But we play to score goals and didn't get any, which is why we are in our current position.
"Dele Adebola is a typical, big Championship striker and he did what he was out there to do by scoring.
"I think we have better quality strikers, but at the moment we are finding it hard to get goals and that has to change."