Charlie Carter’s first goal since August gave Chesterfield’s National League survival hopes a big boost.
Carter scored in only his second appearance since returning from an ankle operation, and post-op infection, at The Hive before Alex Kiwomya wrapped the game up late on.
A third successive win on the road propelled John Sheridan’s side four points clear of the relegation zone.
Sheridan named three changes to the side which lost at home to Harrogate Town at the weekend.
Will Evans, Carter and Tom Denton came into the starting line-up, with Laurence Maguire, Lee Shaw and Marc-Antoine Fortune the trio to miss out in London.
A Barnet corner was cleared only as far as Wesley Fonguck whose stinging shot was straight at Shwan Jalal.
Sheridan’s Spireites struck 15 minutes in as Carter headed home the rebound after Denton nodded against the bar from Ellis Chapman’s cross.
It was almost two when danger-man Denton headed Chapman’s inviting cross over from close range.
The Bees went close to grabbing a leveller just before the break.
Jalal couldn’t hold onto Coulthirst’s long-range strike but gathered in the nick of time before Simeon Akinola could poke home.
Moments later Coulthirst did well to find Akinola under pressure before the winger struck straight at Jalal after cutting inside.
After the restart the visitors had to deal with a barrage of balls in to the box, but it was the visitors who had the chances.
Scott Boden lashed wide as the Spireites searched for a second, before Chapman fired wide from the edge of the box.
Just after the hour Chapman found Boden, who hit his header over.
Coulthirst was the hosts’ biggest threat throughout and he curled a strike off target with just over 20 minutes remaining, before hitting wide after racing down the left.
Substitute Jerome Binnom-Williams could have wrapped things up with a powerful strike but Cousins made a super save to his right.
Kiwomya sealed all three points as he took advantage of a mistake from Cousins to poke home under pressure.
Smith almost beat Cousins deep in stoppage time.
Barnet: Cousins, Johnson, Reynolds, Robson (Mason-Clark, 75), Charles (Boucaud, 69), Taylor, Akinola, Sweeney, Elito (Barham, 45), Fonguck, Coulthirst.
Unused subs: Azaze, Alexander.
Chesterfield: Jalal, Hollis, Evans, Smith, Yarney, Chapman, Weston, Weir, Carter (Binnom-Williams, 56) Boden (Kiwomya, 74), Denton (Reid, 89).
Unused subs: Anyon, Maguire.
Referee: Gary Parsons
Attendance: 902 (170)