His political activities, particularly his support for workers strikes in the state in 1912, caused him to run afoul of employer and he was dismissed, and Cooper subsequently became a journalist and activist within the Labor movement.

Cooper had joined the first majority ALP government in Queensland under T. J. Ryan shortly after his election and became a prominent and well regarded member of the administration.

His reputation was such that he became Assistant Treasurer and Secretary for Public Instruction upon the election of William Forgan Smith as premier of a new ALP government in 1932.

Deputy Premier from 1940 to 1942, he played a key role in preparing Queensland's resources and mobilization for the outbreak of World War II in the Pacific.

He died at his home in Kedron (a Brisbane suburb) in 1949; his State funeral included a procession from St John's Cathedral to the Mount Thompson crematorium.