The Hornets trailed after just 48 seconds, but goals from James Roberts, Jake Elliott and Shamir Fenelon saw the hosts lead 3-1 at half-time.
Wimbledon reduced the deficit on 55 minutes, but substitute Gianluca Botti capitalised on a defensive error to restore the Hornets’ two-goal lead ten minutes later.
The Wombles pegged Horsham back again on 75 minutes, before Botti added his second, and the Hornets’ fifth, with two minutes remaining.
AFC Wimbledon bagged their fourth at the death, but Horsham held on to claim their first win of pre-season.
The Hornets’ next friendly sees them travel to near-neighbours Broadbridge Heath this Saturday.
See some of John Lines’ pictures from the game on this page and the ones linked.
You can read all the latest from the Horsham FC in the West Sussex County Times, out every Thursday.
53. Horsham FC edge out AFC Wimbledon under-19s in breathless nine-goal thriller - the match in pictures
Horsham FC picked up their first win of pre-season after a thoroughly entertaining nine-goal encounter with AFC Wimbledon under-19s. Photo: John Lines
54. Horsham FC edge out AFC Wimbledon under-19s in breathless nine-goal thriller - the match in pictures
Horsham FC picked up their first win of pre-season after a thoroughly entertaining nine-goal encounter with AFC Wimbledon under-19s. Photo: John Lines
55. Horsham FC edge out AFC Wimbledon under-19s in breathless nine-goal thriller - the match in pictures
Horsham FC picked up their first win of pre-season after a thoroughly entertaining nine-goal encounter with AFC Wimbledon under-19s. Photo: John Lines
56. Horsham FC edge out AFC Wimbledon under-19s in breathless nine-goal thriller - the match in pictures
Horsham FC picked up their first win of pre-season after a thoroughly entertaining nine-goal encounter with AFC Wimbledon under-19s. Photo: John Lines