 Cust was educated at Eton College, Trinity College, Cambridge, Haileybury (1840–42) and the College of Fort William, Calcutta, graduating from the last-named institution in 1844.He then worked in the Bengal Civil Services for the East India Company, in Hoshiarpur and Ambala (assistant to the magistrate), in India.He was present at the battles of Mudki, Ferozeshah, and Sobraon in 1845-46, where his superior Major George Broadfoot was killed and at the close of the Sikh campaign he was placed in charge of a new province in the Punjab.