Horsham became a founder member of the West Sussex County League in 1886, 4 years after helping found the Sussex County Football Association, and finished their debut season in second place behind champions Southwick before celebrating back-to-back title success between seasons 1899/1900 and 1901/02, also capturing the Sussex Royal Irish Rifles Cup in 1900.

In 1904 the Club made Queen Street it's permanent home where they would remain for more than 100 years but it would be almost a quarter of a century before they could celebrate another championship triumph when their 1925/26 win finally saw them accepted in to the County League and the start of a golden era for Horsham FC.

The 1930s was a decade of near total domination of the County scene with the league title secured on no less than six occasions, the Sussex Royal Ulster Rifles Cup seven times, and the Senior Cup twice before the outbreak of War brought a temporary halt to the Club's success. However, upon the League's resumption in 1946, Horsham landed the title again as well as 4 more cup successes. During this time the club also reached the 1st Round Proper of the FA Cup for the first time where they shocked Tommy Lawton's Notts County side by taking a first minute lead, only to bow out 9-1.

Horsham bade farewell to County League football in 1951 when they joined the Metropolitan League