Barnsley took the lead after just three minutes when a cross sailed over Lewis Carey’s head for Nicky Cadden to tap home.
The Tykes doubled the advantage on ten minutes when the Hornets again failed to deal with a cross and McAtee was there to slot home at the back post.
McAtee made it 3-0 after 26 minutes after Jordan Williams delivered a delightful ball into McAtee who saw Carey off his line and produced a delightful chip.
But Horsham were simply excellent in the second half and were unfortunate not to get a goal their attacking play so richly deserved.
But who stood out for Horsham? Here are Sam Pole's ratings from the game.
9. James Hammond
James Hammond was the pivot in midfield for the Hornets. His pressing in the second half was too much for the Barnsley midfield to handle at times and provided the spark that the Hornets needed coming out of the break. Photo: John Lines
10. Shamir Fenelon
Fenelon, although often isolated up front, did much of the leg work. His link up play between the midfield was good despite not having any clear-cut chances on goal before being replaced by Kadel Daniel just before the hour mark. Photo: John Lines
11. Lee Harding
Harding was a thorn in the side of the Barnsley defence on the right hand side his tricky play as well as his defensive work in the late second half capped off an good display for the Hornets winger. Photo: Photo: Ben Roberts Photo
12. Dan Ajakaiye
Ajakaiye was lively throughout up front, his pace was constant threat and he almost pulled one back for the Hornets on 52 minutes after beating a couple of players, fired just wide. Ajakaiye was again close to bringing one back for the Hornets after Harding pulled it back across goal but the Hornets striker could not apply the fninishing touch. Photo: Photo: Ben Roberts Photo