Barnet picked up their first points since their return to the Football League as Luke Gambin ran rings around Northampton.
The midfielder was introduced on the hour mark and instantaneously changed the game, which had more resembled a head tennis match than football, with his pace and industry.
He was involved for they key moment of the game as he embarked on a run through the middle before finding John Akinde. The striker was brought down in the box by Jason Taylor, who was sent off for denying an obvious goalscoring opportunity. Akinde rolled the ball home from the spot.
The Bees had been one paced and one dimensional in the first half, aimlessly pumping long balls forward, which were coming straight back at them.
Northampton had missed chances with Dominic Calvert-Lewin denied by goalkeeper Graham Stack but were hit on the counter on 90 minutes as Akinde beat Smith to a lobbed ball over the top. He rounded the goalkeeper and slid the ball home to secure the victory for Barnet.
