The deadlock was broken when Matthew Sparrow was felled in the Torquay box, prompting the referee to give the away side a penalty. Winnall slotted the ball into the bottom right corner to gift his team the lead.
The beleaguered home side could have pulled it back when they won a penalty of their own at the end of the first half, but Dale Tonge's effort was saved by Slocombe.
Hakeeb Adelakun, on for Michael Collins at half time, stung the hands of Martin Rice with a free kick four minutes into the second half, before nearly picking out Winnall with a defence-splitting pass just a minute later.
The hosts won two corners in the space of two minutes on the hour, but they weren't able to capitalise, with the Iron clearing the danger.
The sting was taken out of the game in the 70th minute when Hawley had to be stretchered off following an injury, but Scunthorpe were able to keep the home side at bay for the duration of the game.