The Yeltz were cheered on by a travelling crowd of over 650, including eight supporters' coaches, on an emotionally-charged afternoon and soon settled to hold their own before claiming their third National League scalp away from home in their thirteenth FA Trophy game of the season.
A heavy pre-match shower left a sticky midfield area and in the first action goalkeeper Brendon Bunn grabbed a 3rd minute short corner from Barnet's Elliott Johnson.
Halesowen's James Bowen then burst forward from a Barnet mistake to see his seventh minute shot blocked and Ahmed Ali played the ball forward for Simeon Cobourne and Montel Gibson to combine and force the first Yeltz corner after 12 minutes. A minute later Ali headed on Bowen's throw-in for Cobourne to be blocked again.
There was little to choose between the teams in an end-to-end first half and Brendon Bunn easily collected a flick from Josh Walker after Mauro Vilhele crossed from the right in the 21st minute.
In difficult blustery conditions, impressive Bowen then sent in a low 32nd minute cross from the left.
Walker jinked into the penalty area after 33 minutes but fired a low shot at Brendon Bunn and then Ali, Rob Evans and Paul McCone made good defensive clearances.
As Halesowen pressed forward, Ricardo Santos cleared over his own bar from Cobourne's 40th minute cross and Josh Hawker's edge of the area free-kick was cleared after Cobourne was brought down a minute later.
In the final minute of the first half, Simeon Akinola found space in the Halesowen area after Evans slipped but he shot weakly at Brendon Bunn on the turn.
At the start of the second period David Tutonda screwed wide from a right wing cross and a 56th minute shot by Cheye Alexander was collected by visiting goalkeeper Brendon Bunn.
Walker then shot wide on two occasions and Akinola went close again as Barnet stepped up a gear.
The Bees then grabbed a 65th minute lead when Johnson combined with Tutonda and his cross from the left was turned home by Walker.
But Halesowen were undeterred and stuck to their task as a 70th corner was punched clear by goalkeeper Scott Loach and Ali headed wide from Robbie Bunn's 81st minute cross.
The Yeltz were rewarded with an 85th minute equaliser after Robbie Bunn jinked down the right and his cross skimmed the head of substitute Luke Yates before drifting into the far corner of the net.
An 88th minute shot from substitute Jamie Molyneux earned another corner and Molyneux did well again a minute later as Cobourne wriggled through to see his shot deflected over the bar.
Halesowen finished the ninety minutes on top with Robbie Bunn's 90th minute free-kick from the edge of the box blocked.
Controversy reigned in the third minute of injury time when Cobourne was brought down and a free-kick was awarded on the edge of the area - despite early indications of a penalty - and the low free-kick from Evans was deflected into the arms of Loach.
At the start of extra-time Robbie Bunn delivered a dangerous cross and Barnet forced two corners.
An Alexander cross was then turned away well by Jamie Ashmore and Brendon Bunn dived to make a good save from home substitute Medy Elito from fellow substitute Wesley Fonguck's 107th minute cross.
But the visitors finished strongly as an 108th minute effort from substitute Jack Holmes was turned away for a corner.
The mighty Yeltz then amazed the Hive with a winning goal in the 109th minute when Ali headed on Robbie Bunn's corner for Cobourne to touch home on the goal-line.
Glorious Halesowen almost added further goals when Holmes went close twice and Molyneux also broke forward and his cross just evaded Holmes in the 120th minute at the end of an incredible performance.
Barnet: Scott Loach; Cheye Alexander, Elliott Johnson, Callum Reynolds, Ricardo Santos, James Dunne (c), Simeon Akinola (Medy Elito 82), Mauro Vilhete (Wesley Fonguck 56 (Charlee Adams 112)), Josh Walker, David Tutonda (Dan Sparkes 112), Dan Sweeney. Subs Not Used: Rihards Matrevics (GK), Harry Taylor, Xander McBurnie.
Halesowen Town: Brendon Bunn; Jamie Ashmore, James Bowen (Luke Yates 68), Rob Evans, Paul McCone (c), Jordan Piggott, Ahmed Ali, Robbie Bunn, Montel Gibson (Jamie Molyneux 58), Simeon Cobourne (Ash Sammons 113), Josh Hawker (Jack Holmes 76). Subs Not Used: Lewis Wright, McKauley Manning, Bradley Catlow (GK).
Referee: Adrian Quelch Attendance: 1,483