He enlisted in the British Army as a private soldier in November 1941, during the Second World War, and from 1941 to 1944 served as a lieutenant in the Intelligence Corps on the staff of the South East Asia Command at Kandy, Ceylon.

Brooke was appointed as heir apparent with the title of Rajah Muda of Sarawak on 25 August 1937 and was granted the personal style of His Highness.

Having been responsible for administering Sarawak between 1939 and 1940, in the absence of the Rajah, he was deprived of his styles and titles on 17 January 1940, then dismissed and expelled from the state in September 1941, following a dispute with his uncle, Rajah Vyner Brooke, over his marriage to a commoner, Kathleen Hudden, sister of a Sarawak government official.

In 1946, Rajah Vyner ceded Sarawak to the British Colonial Office, in exchange for a sizeable pension for him and his three daughters.

Anthony Brooke, the designated heir, initially opposed the cession to the Crown, along with a majority of the native members of the Council Negri (Parliament).

A five-year campaign in Sarawak followed, aimed at revoking the country's new colonial status, in part directed by Brooke from his house in Singapore.