He entered the East India Company's Bengal civil Service as a Writer in 1826, having displayed from an early age a great proficiency in Asian languages and dialects. On 4 January 1827, Trevelyan was appointed assistant to Sir Charles Theophilus Metcalfe, the commissioner at Delhi, where, during a residence of four years, he was entrusted with the conduct of several important missions. For some time he acted as guardian to the youthful Madhu Singh, the Rajah of Bhurtpore. He also worked to improve the condition of the native population. He abolished the transit duties by which the internal trade of India had long been fettered. For these and other services, he received the special thanks of the governor-general in council. Before leaving Delhi, he donated personal funds for construction of a broad street through a new suburb, then in course of erection, which thenceforth became known as Trevelyanpur.