Founded as the Sidcup County School for Boys in 1931 to meet the lack of secondary schools in the newly urbanised town, it was initially opened at 27 Station Road, with the position of first headmaster being given to C. R. McGregor Williams. In 1935, the school began moving into a purpose-built site at Critall's Corner, Footscray, being renamed Chislehurst and Sidcup County School in 1938. Damaged during The Blitz, after the culmination of the Second World War, reforms implemented as a result of the Education Act 1944 led to the local decision that the institution would become a grammar school and that it would relocate to a new, larger building on Hurst Road in the Lamorbey area of Sidcup, a move that took place after the resignation of McGregor Williams in 1954.