He received the chairmanship of Australian Paper Mills (which his father had founded in 1882), and also managed the William Brookes & Co. pastoral holdings in Queensland and Western Australia.

In 1929, Prime Minister Stanley Bruce appointed him Commissioner-General to the United States, working within the British embassy in Washington, D.C. as Australia did not have separate diplomatic representation at that time.

From 1932 to 1939, Brookes served on the board of the Australian Broadcasting Commission, including as vice-chairman; he was offered the chairmanship, but refused it due to his inability to secure a guarantee of independence from government control.

[1] As a distinguished alumnus, Brookes served on the University of Melbourne's council from 1933 to 1947, where he was particularly active on its finance committee.

[4] Always in the background of anti-Labor politics, he was a bitter and tireless campaigner against trade unions and the Catholic Church, which he saw as an instrument of Irish subversion.

In the 1920s he used his considerable personal wealth to finance a private intelligence unit, which gathered information on trade unionists and Catholics.

[5] He was an advocate of profit sharing, adult education, and equal pay for women, all of which he introduced in his own companies, and campaigned for better working conditions in mines to eradicate miner's phthisis.