The 2nd Marquess's public career centred around India, first as Governor of Bengal during World War I, and subsequently as Secretary of State for India in the 1930s. Although a member of the Conservative Party, his belief was that Indians should be allowed to take ever-increasing responsibility for the government of the country, culminating in Dominion status (the status enjoyed by Canada, Australia, and other formally independent members of the British Empire). He played an important role in the protracted negotiations which led to the 1935 Government of India Act, which came as close as was possible, given the implacable opposition of Winston Churchill and the "diehards" to anything that might imperil direct British rule over India, to implementing these ideals, and was ideally placed as Secretary of State to implement them — although the two Viceroys with whom he served, Lords Willingdon and Linlithgow, were rather less idealistic than he. In the event Willingdon and Linlithgow were proved right when the Congress Party won the 1937 Provincial elections, much to the dismay of Zetland. Zetland's term as Secretary of State — and the experiment with democracy represented by the 1935 Act — came to an end with Churchill's assumption of the Prime Ministership in 1940: Zetland then offered his resignation, feeling that his ideas and Churchill's regarding India were so different that "I could only end by becoming an embarrassment to him."
Zetland was also a noted author: Rab Butler, who served under him in the India Office, records how, asking how he could understand better his chief's thinking about the future of India, received the answer: "Read my books!"