Walter Figueira’s early strike and Tom Beere’s free-kick on the stroke of half-time gave the Robins a commanding lead.
Hornets midfielder Lucas Rodrigues reduced the deficit early in the second half but Carshalton substitute Calvin Ekpiteta made sure of the three points late on.
The defeat sees early pacesetters Horsham drop from the Premier’s summit to third, albeit just a point behind new leaders Bognor Regis Town and second-placed Lewes.
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 1, Carshalton Athletic 3 - the match in pictures
Action from Horsham's home defeat to Carshalton Athletic. Picture by John Lines Photo: John Lines
2. Horsham 1, Carshalton Athletic 3 - the match in pictures
Action from Horsham's home defeat to Carshalton Athletic. Picture by John Lines Photo: John Lines
3. Horsham 1, Carshalton Athletic 3 - the match in pictures
Action from Horsham's home defeat to Carshalton Athletic. Picture by John Lines Photo: John Lines
4. Horsham 1, Carshalton Athletic 3 - the match in pictures
Action from Horsham's home defeat to Carshalton Athletic. Picture by John Lines Photo: John Lines