William Reid was born at Falkirk, Scotland, in 1889 to William and Elizabeth Reid, and emigrated to Western Australia as a young man. Reid was working as a labourer before enlisting with the Australian Imperial Force (AIF) on 22 July 1915. He was allotted to the 16th Infantry Battalion as a private with the service number 2924. After several months in camp he and the other reinforcements left Fremantle aboard HMAT Hororata on 5 October 1915.

The reinforcements first went to Egypt where, having since evacuated from the Gallipoli peninsula, the Australian infantry were preparing for the move to the Western Front. In early June 1916, Reid and the 16th Battalion sailed for France. The battalion took part in the fighting at Pozieres in August 1916. Later that year, on 20 December 1916, the 16th Battalion band was officially established, and Reid became a member. He began keeping a small black notebook detailing the history of the band and its movements. Reid recorded band related events, such as playing programmes, playing the Battalion out of the line, the practice sessions the band held, and the concerts they played. He also recorded events at the Front, such as a route march of 100 kilometres, German planes being shot out of the sky, inter-battalion sporting matches (including one where German shells killed several Battalion soldiers), trench digging while being bombarded by gas shells, pulling mules out of canals, and the funeral for a soldier who drowned on the Somme.

Reid remained with the battalion throughout 1917 as it fought in northern France and Belgium, in particular suffering heavy losses at Bullecourt in April. In late March 1918, shortly after Reid returned from leave, the Germans launched their last great offensive, the 16th Battalion doing its part to halt the advance. It would later take part in the fighting near Amiens in August. In early September, Reid was detached to the 4th Army Harvesting Party at Allonville and wrote about loading 10 General Service Wagons with oats on one occasion. He rejoined the 16th Battalion early the next month. With the cessation of hostilities on 11 November 1918, Reid was allowed three weeks leave in France. He eventually left France in early April 1919 and returned home aboard the troopship Marathon on 19 April 1919. Reid arrived back in Western Australia around six weeks later and was formally discharged on 24 July 1919.