The Hornets dominated the first half, and lead at the break courtesy of Ricky Aguiar’s excellent finish following a wonderful team move.
But a poor 15 minute period in the second half cost the hosts as Chesham, who had not won an away game since late October, hit back to take the three points.
A corner was turned home by Matt Lench for the equaliser on 63 minutes before Karl Oliyide’s 74th minute goal ended the Hornets’ 17-game unbeaten run.
The loss to the Generals saw Horsham drop to sixth in the National League South table.
The Hornets will look to get back to winning ways when they visit fourth-placed Dorking Wanderers on Boxing Day.
See pictures by John Lines from Horsham v Chesham United on this page and those linked – and get the latest from Dominic Di Paola in the West Sussex County Times, out on Thursday.
1. Horsham FC's unbeaten run ended by Chesham - the match in pictures
Horsham FC tasted defeat for the first time in 91 days following a surprise 2-1 National League South loss to Chesham United. Photo: John Lines
2. Horsham FC's unbeaten run ended by Chesham - the match in pictures
Horsham FC tasted defeat for the first time in 91 days following a surprise 2-1 National League South loss to Chesham United. Photo: John Lines
3. Horsham FC's unbeaten run ended by Chesham - the match in pictures
Horsham FC tasted defeat for the first time in 91 days following a surprise 2-1 National League South loss to Chesham United. Photo: John Lines
4. Horsham FC's unbeaten run ended by Chesham - the match in pictures
Horsham FC tasted defeat for the first time in 91 days following a surprise 2-1 National League South loss to Chesham United. Photo: John Lines