West Bromwich Albion surrendered their 100 per cent home record. It was a result that infuriated the Albion fans who called for the head of manager Bryan Robson at the final whistle.
Harrold pounced seven minutes from time to earn Southend their first point on the road this season.
But their share of the spoils owed everything to an inspired display from goalkeeper Daryl Flahavan. He was a one-man rearguard at times against Albion and his heroics paid off in style when Harrold converted Steve Hammell's corner.
Albion only had themselves to blame for not taking all three points as they dominated the game for long spells.
But all they had to show for their efforts was a 30-yard wonder goal from striker Nathan Ellington in the 61st minute.
For once, Flahavan was powerless when Ellington rolled his right foot around the ball 30 yards out in the 61st minute and curled home his second goal of the season.
It should have been enough to earn Albion - who hit the post through Kevin Phillips in the 81st minute - a fourth straight home win.
But their earlier failings in front of goal came back to haunt them as they paid the penalty for some poor defending when Hammell whipped in his right-wing corner.
Albion had been desperate to pick up three points as it has only been their home form that has kept them in touch with the early season promotion contenders.
A dismal haul of just two points on the road had put extra focus on their home form and it was a pressure they could not live with against Southend.
But, had it not been for Flahavan, Albion would have been home and dry by half-time.
He really did stop Albion turning the game into a rout as he pulled off no fewer than six top-class saves in the opening 45 minutes.
The heroic Flahavan denied Zoltan Gera, Ellington and Phillips with a series of stunning saves.
Backed by the form of Flahavan, Southend noticeably grew in confidence at the start of the second half and they should really have broken the deadlock when Phillips was robbed by Jamal Campbell-Ryce on the halfway line.
He surged forward and exchanged passes with Freddy Eastwood only to fire wide when well placed.
Ellington punished him with what looked destined to be the matchwinner, but Albion paid the price for failing to pay attention to Harrold when Hammell floated over his cross.