Matthew Johnson fired home from a Blues corner to give the away side the lead on 39 minutes.
Frank Merrifield hit what proved to be Bishop’s Stortford’s winner five minutes into the second half.
The result sees Horsham drop to fifth, but just four points adrift of early pacesetters Hornchurch and Potters Bar Town.
The Hornets will look to get back to winning ways tonight when they visit seventh-placed Enfield Town (7.45pm).
See some pictures by the club's photographer John Lines on this page and the ones linked.
Get the latest from the Camping World Community Stadium in the West Sussex County Times - out every Thursday.
1. Horsham 0, Bishop's Stortford 2 - the match in pictures
Action from Horsham's 2-0 home defeat to Bishop's Stortford in the Isthmian Premier on Saturday Photo: John Lines
2. Horsham 0, Bishop's Stortford 2 - the match in pictures
Action from Horsham's 2-0 home defeat to Bishop's Stortford in the Isthmian Premier on Saturday Photo: John Lines
3. Horsham 0, Bishop's Stortford 2 - the match in pictures
Action from Horsham's 2-0 home defeat to Bishop's Stortford in the Isthmian Premier on Saturday Photo: John Lines
4. Horsham 0, Bishop's Stortford 2 - the match in pictures
Action from Horsham's 2-0 home defeat to Bishop's Stortford in the Isthmian Premier on Saturday Photo: John Lines