The Athenian League was founded in 1912, reforming again in 1920 after the intervention of WWI. Its membership fluctuated but generally numbered between 12 and 16 clubs until 1963 when sides from the Corinthian and Delphian Leagues were absorbed to make 3 divisions (those from the Corinthian League forming Division One and the Delphian Division Two with the existing Division renamed the Premier Division). Horsham were among those involved in the 'new' Division One but made little initial impact, only Epsom & Ewell keeping them off the foot of the inaugural table, and a 4th placed finish the following season was overshadowed when the club suffered relegation for the first time in their history in 1965/6.

Under coach Pat Tobin, Horsham finished third, third and fifth but it was the appointment of Roy Osborne in 1969 that saw silverware finally come the club's way with the clinching of the Division Two championship in 1969/70 and, three years later, the Division One title was won when Staines Town were beaten in to second place. Other notable feats during this time saw the club reach the First Round of the FA Cup for only the second time, setting a Queen Street record attendance of 7,134 for the 1966 visit of Swindon Town, while the disappointment of three consecutive Senior Cup Final defeats was erased with victory over Whitehawk in 1972. Neighbours Horsham YMCA were thumped 10-1 at Gorings Mead in November '71 to establish a club record FA Vase victory and it was also during this time that Horsham Football Club became known as 'The Hornets' when the nickname was adopted thanks to a competition run amongst the club's supporters.