Josiah Allen was born at Gin Gin, Queensland, on 11 February 1890. He was considered a skilled marksman and horseman, and was a member of the Gin Gin Rifle Club. The 26-year-old farmer and grazier enlisted for service with the Australian Imperial Force on 5 July 1916, as did his brothers Ernest and James Edward. The three brothers departed Sydney with reinforcements for the 49th Infantry Battalion aboard HMAT Ceramic on 7 October 1916.

Having been sent to the Western Front, Allen wrote two letters to his brother William back in Australia in which he responded to news from home and described the conditions at the Front. Grateful for the food packages from home he despaired at the continual rain and cold weather on the Front. On 7 June 1917, Allen and his two brothers fought at the Battle of Messines. It was on this day that Josiah Allen, initially listed as missing, was later confirmed to have been killed in action, along with his brother James. His body was never recovered, and today Josiah Allen is commemorated on the Ypres (Menin Gate) Memorial, Belgium.