After leaving school at the age of 16, he initially trained to be a priest at Corpus Christi College, Melbourne from 1939 but after the events of Pearl Harbor joined the Navy.
He practiced predominantly in criminal law, but also appeared in coronial inquests, government inquiries, personal injury and medical negligence matters.
[5] He oversaw the preparation of a report commissioned by the Australian Government, which recommended the changing of the policy of assimilation to one of multiculturalism.
Diamond appeared as a medical witness in a number of homicide cases in which he successfully used novel approaches to argue the defendant's mental state as a mitigating circumstance.
[8] During the 1980s, Galbally represented the Painters and Dockers Union in the Costigan Commission and was involved in the appeal to stop the Barlow and Chambers execution.