Michael Hartnett joined the Royal Munster Fusiliers in 1915. His motive in doing so was partly to gain experience which could be useful in the struggle for Irish freedom in which his family had a passionate belief.
In late 1917, he transferred to the fledgling Royal Flying Corps. His plane was shot down over Flanders, in 1917, just before his twenty-first birthday. He survived the crash, but died later in a German hospital. He was buried in a German graveyard under the inscription "unknown English pilot". His remains were removed to Harlebeke Cemetery,in Belgium, and at this point his identity was established.