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.