Charlie Hester-Cook’s 12th minute header saw off the Scholars on Tuesday night in a 0-1 loss at the LA Construction Stadium against Horsham.
The result sees Horsham move five points behind Bar, who sit in 12th place in the Isthmian Premier Division.
Joe Boachie had an early penalty shout for Sammy Moore’s side when he was brought down in the box, but the referee waved the appeals away.
The Hornets then took the lead early on with what proved to be the only goal of the game as Hester-Cook got to a cross before Hafed Al Droubi in goal for The Scholars to head home for Horsham.
Jacques Kpohomouh prevented Dominic Di Paola’s side from doubling their lead after Isaac Ogundere’s pass back to his keeper was intercepted by Eddie Dsane.
The 25-year-old rounded Al Droubi, but Kpohomouh was on hand to make a vital block off the line to keep the score at 0-1.
The ball was hooked off the line again from a corner around the half an hour mark, with the clearance falling to Hester-Cook.
He was unlucky not to score his second of the day on the volley, as his effort flew just over the bar.
Bar saw a second penalty appeal turned down when a cross seemed to hit a Horsham hand on its way through the box, but nothing was given by referee Michael Smith.
Lee Harding had the ball in the net for Horsham in added time with a tap-in, but the linesman’s flag was raised and the goal was disallowed.
Half time score: Potters Bar Town 0-1 Horsham
Tom Richards made an important interception to stop Corey Parchment turning in Boachie’s cross to start the second half.
Lucas Rodrigues found himself in on goal but chose to pass rather than shoot, costing himself a goal and Horsham a second on the night.
There was very little action towards the end of the second half as Bar ran out of time to equalise.
The Scholars almost got a late leveller, but Parchment’s cross was just too far away for any player in a Maroon shirt to reach.
Full time saw Potters Bar condemned to a disappointing defeat against Horsham at home.
The Scholars face Merstham at home on Saturday where they will look to bounce back from the loss.