Michael was born in the Walgett district of NSW to Michael and Margaret Mannix. At the age of 19 he enlisted at Armidale in February 1916 giving his address as Leigh, Dorrigo NSW where his parents lived. The previous year he had been visiting ‘the northern part of the State’.
Michael joined the 33rd Battalion, 1st Reinforcement and left Australia on HMAT A74 Marathon in May 1916. After arriving in England in July he was sent to France in September as part of the reinforcement for the 53rd Battalion. In early December he was wounded in action, returned to England, and admitted to Norfolk War hospital with a gunshot wound to his head. The following month he was transferred to the 1st Auxiliary hospital at Harefield.
In February 1917 he requested leave to go home as his father was dying and his mother had young children to care for. Eventually his leave was granted. He arrived in Sydney in July and was discharged.
Michael died in 1968. It is believed that he is the M. Mannix whose name is recorded on the Yetman Honor Roll.