In 1892 Rosenthal joined the Geelong Battery of the Victorian Militia Garrison Artillery as a gunner, but left the service three years later on moving to Melbourne. In 1903 he was commissioned second lieutenant in the Militia Garrison Artillery. He transferred to the Australian Field Artillery in 1908 where he was promoted as major. In 1914 he became commanding officer of the 5th Field Artillery Brigade. Thus before the war he was established as a soldier as well as a professional architect.