William was the 10th child of Patrick and Margaret Breen and a younger brother of Clement. He enlisted in Adelaide in January 1916 and stated that his sister was his next of kin, despite his parents still being alive. During the later part of 1916 he spent time in hospital in England with pleurisy before once again returning to ‘the front’. On 11 May 1918 he faced a Field General Court Martial charged with desertion—in that he at Ypres on 23 September 1917 after being warned for duty in the front line absented himself until apprehended on 28 February 1918. He was found guilty and sentenced to 10 years penal servitude. This was later reduced to 3 years and soon after was suspended allowing him to re-join his unit on 18 July 1918. His service record unfortunately does not give any explanation for his actions.
William Breen later returned to Tasmania. He married Beulah and together they had two sons William passed away on 25 June 1937 and is buried in the same grave as his brother Clement.