The Blades haven’t progressed past round four since their run to the semi-finals in the 2013/14 season but Sharp’s 80th-minute penalty saw them move into the next round with consecutive 1-0 wins.
United boss Chris Wilder said: “It wasn’t a classic game. We just needed to lift our standards and be a bit more aggressive in the second half and drive the game forward a little more.
“I thought the introduction of Ched (Evans) gave us a little spark, and (Chris) Basham as well.
“We changed the shape at half-time as well just to get a bit more control of the football, which I think it did and we just edged a tight game.
“It was two teams that are very evenly matched, in league position. They made a few changes and we made a few changes, so I think both managers will be delighted that it has been decided on the day.”
Meanwhile, it was disappointment for the Lilywhites, as they came into the game with only one loss in their last seven FA cup ties away from home but instead suffered their first defeat to United in the FA Cup since 1936.
George Baldock, one of only four players to retain his place in the Blades’ starting XI, started brightly, getting forward on the right flank and bursting past several defenders, but could not produce a final ball to worry the back four.
Off the back of signing a new two-and-a-half-year deal with the Blades on Friday, Mark Duffy grew into the game. It was his pass that sent Baldock to the byline to drive in a low cross which Clayton Donaldson could only direct into the side-netting.
Preston winger Daryl Horgan tried to create something himself, weaving down the left-hand side and getting into the box, however centre-back Richard Stearman made a fantastic sliding tackle to deny him any chance of getting a shot off.
After a first half with no shots on target, the second 45 started off better when Duffy’s low cross was met by Donaldson at the front post, however similar to the first half, he could not hit the target.
Baldock and Duffy linked up well again down the right, before the latter sent a teasing cross into the box. Daniel Lafferty met the delivery with a well-struck volley, forcing Declan Rudd to make his first save of the afternoon, stretching to tip the ball over the bar.
With the reality of a midweek replay looming, both teams made double changes and upped their intensity going into the final 15 minutes.
After stalemate in the battle between the two captains, Sharp finally got one over his opposing skipper at the death, as Tom Clarke’s last-ditch attempt to tackle the striker resulted in referee Graham Scott awarding a penalty. Sharp then stepped up to smash the spot-kick to the right of Rudd, sending him the wrong way.
The away side pushed for an equaliser late on, but goalkeeper Simon Moore kept his clean sheet, impressively denying Daniel Johnson and Louis Moult.