Born on 24/2/1821 to Reverend Henry Cockayne Cust, he was educated at Eton College, Trinity College, Cambridge, Haileybury (1840–42) and the College of Fort William, Calcutta, graduating 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. There he filled in succession every office in the judicial and revenue departments across Punjab, and was rapidly promoted until 1867, when he resigned and returned to England, after having been a member of the Viceroy's Legislative Council and Home Secretary to the Government of India in 1864–65. Cust returned to England briefly on furlough and returned to work in Benares and Banda. Cust was called to the bar at Lincoln's Inn on 13 August 1857. He returned to India and served in Lahore until 1864. He later joined the legislative council and acted as home secretary to the government (1864–65). In 1867 he decided to retire from Indian service. After retiring to England Cust devoted himself to scientific research, philanthropy, and magisterial and municipal duties, declining reappointments in India. He was a member and officer in many scientific, philanthropic, and religious societies and a prolific writer. He was one of the few Victorian intellectuals to oppose the racist theories popular at the time.