Hendry notched his sixth goal of the campaign as the Perth outfit came from behind to secure a 2-1 win over Ladbrokes Championship side Ayr.
That was cancelled out in the 18th minute by an Aaron Muirhead own goal under pressure from Hendry. The 22-year-old striker was claiming it but TV pictures proved that Muirhead, who was sent off just eight minutes later for a lunge on Liam Craig, headed past his own goalkeeper Ross Doohan.
But there was no disputing who got Saints' winner as less than a minute after Muirhead received his marching orders, Hendry headed home a Drey Wright cross.
All of Hendry's previous five goals this season have come when he has been a substitute and Wright was delighted his decision to start him paid off.
Wright said: "Callum was claiming the first goal. Their young goalkeeper was a bit unlucky as the ball dies in the wind, I think he was coming for it and he ended up a bit short.
"We didn't just wake up in the morning and think we're going to play Callum. We looked at the games coming up and thought this game that we might be able to dominate possession although it didn't look like that at the very start.
"Callum, at this minute, comes alive more in the box. I was pleased for him.
"Our second goal was a great header by Callum. It was a great ball in by Drey, a great cross and no matter what the conditions are it was a great goal."
Ayr player-manager Kerr admits Muirhead's dismissal affected their game plan.
And playing into the wind in the second half, as Storm Ciara started to batter the country, hindered their attempts at a comeback.
Kerr said: "We worked on getting Aaron more central and having runners off him and try to have a go. We started well and it's disappointing as all that stopped when we went down to 10 men. After that we had to take a creative player off.
"Young Luke (McCowan) came off and he was going to get chances and put on another defender.
"So it was never going to be easy.
"In the second half we wanted to turn it into a cup tie, which if we had the wind we might have been able to do.
"But we were against the wind in the second half and we couldn't put them under pressure.
"Going down to 10 men spoiled a good cup tie and the weather did add to that."