Two and a half of the additional four minutes had been played, with this often entertaining encounter seemingly heading for a goalless draw - but then Symes struck his third goal of the season to secure the home side only their second victory in ten league matches.
While it was tough luck on high-flying Barnet, who looked set to leave with a point, Town just about deserved their victory after dominating the final 20 minutes, although it did appear their pressure would be in vain until Symes ensured all three points.
Despite not scoring until the dying moments, Shrewsbury had numerous chances throughout the game, with the wastefulness of striker Stuart Nicholson almost costing them dear.
On three occasions and twice in quick succession during an evenly contested first half, Nicholson had only Barnet keeper Lee Harrison to beat, but was thwarted on each occasion.
The visitors created the best of their opportunities in the first half with the lively strike pairing of Liam Hatch and Adam Birchall keeping the home defence busy - with Hatch having a goal ruled out for offside, while Birchall forced a diving save from Shrews goalkeeper Glyn Garner.
After the break Shrewsbury gradually gained the upper hand and with centre-forward and top scorer Dave Hibbert unfortunate not to score on a number of occasions, it seemed the afternoon would end in frustration for the home side as Barnet repelled their attacks and with time almost up a draw looked the only possible outcome.
But right at the death good work from second-half substitute Marc Pugh created the opening for Symes - who came on with just ten minutes to go - and he was able to drill the ball under Harrison and into net to clinch a welcome win for Gary Peters' men.