The field goal came after Carolina had tied the game at 16-16 with 1:54 remaining on a 29-yard pass from third-string quarterback Brett Basnaez to Chris Hannon.
Instead of going for a two-point conversion, Carolina head coach John Fox had John Kasay kick the PAT, tying the game and giving the ball back to the Steelers with 1:47 remaining.
Dennis Dixon, a fifth-round draft pick, completed four passes for 31 yards and also scrambled for another 11 yards to set up Reed's game-winning attempt as the Steelers ended the preseason at 3-1.
A botched snap by the Steelers on their opening possession set up Carolina's first score.
Center Justin Hartwig, named the starter by Tomlin earlier this week, snapped the ball too early to quarterback Ben Roethlisberger, who was in shotgun formation. The ball sailed over Roethlisberger's head and the quarterback fell on it at the Pittsburgh 4.
Following a punt, Carolina – which did not play its starters – took over at its own 48 and turned the miscue into a 32-yard John Kasay field goal and a 3-0 lead.
The Steelers answered on their next possession, however, getting a 26-yard run and a 37-yard pass interference penalty to set up a six-yard TD run by fullback Carey Davis.
That was it for the first-team offense, as Roethlisberger and company watched from the bench from that point. Roethlisberger threw two passes, but both were negated by penalties. Parker had four carries for 34 yards.
The first-team defense saw action in just two series as well, with the second ending on an interception by Bryant McFadden. McFadden returned the interception to the Carolina 41 to set up a 41-yard Reed field goal that put the Steelers ahead 10-3.
Reed added a second field goal, this one from 49 yards out, midway through the second quarter to give the Steelers a 13-3 lead.
A fumble by Steelers rookie running back Rashard Mendenhall, his third in the past two games, ended Pittsburgh's next possession at midfield.
Mendenhall finished the game with 79 yards on 21 carries, giving him 212 yards rushing on 54 attempts in the preseason.
The Panthers did not take advantage of that miscue, but did on one by punter Paul Ernster, who is locked in a battle for the Steelers' punting job with Mitch Berger.
Ernster shanked a punt just 22 yards to the Carolina 41 and the Panthers moved into range for Kasay to boot a 32-yard field goal to cut the score to 13-6 at the half.
Mendenhall carried the Steelers into field goal range by himself on Pittsburgh's opening possession of the second half, taking the ball on eight consecutive plays and gaining 47 yards to set up a 41-yard Reed field goal for a 16-6 lead.
Carolina added a 48-yard Kasay field goal early in the fourth quarter to time the lead to 16-9.