The veteran forward was presented with the opportunity to equalise in the 85th minute, three minutes after Scunthorpe had gone into the lead for the first time.
Chris Basham put the Blades in front shortly after the half hour mark but ten minutes after the restart, the hosts levelled through Josh Morris.
Basham then saw red for rash challenge on Morris on with 23 minutes remaining and Scunthorpe looked to have made him rue when Duane Holmes netted late on.
But Sharp was the hero of the day – like he has been on so many occasions during his career – when he kept his cool to net a penalty with five minutes remaining.
The draw sees Blades drop out of the play-off places into seventh but the four-point gap to the top remains in place.