William Mansfield enlisted at Melbourne Town Hall on 16 May 1916. He was a married man, living with his wife Adelaide at 535 Victoria Parade, East Melbourne. They had married around the same time that he had enlisted. He was 47 years old at the time and described his occupation as labourer, and on a second form, cook. His was, by religion, Church of England.

He was sent to Broadmeadows for training and placed as one the 15th Reinforcements to the 21st Battalion. The 21st had been raised at Broadmeadows as part of the 6th Brigade, 2nd Division, with recruits drawn from all over Australia. They arrived in Egypt in June 1915 and went through further training in Egypt before being sent as reinforcements to Gallipoli in late August. On the way to Anzac Cove, their ship, HMT Southland, was torpedoed by the German submarine UB-14 near Lemnos and all on board had to abandon ship. They eventually arrived at Anzac Cove, Gallipoli, on 7 September following the failure of the Allied push. This meant that they undertook mainly defensive duties along the Australian line until 15 Decemner, when the allies withdrew.

On their return to Egypt, the 21st Battalion took part in the defence of the Suez Canal, as well as undergoing further training On mid-1916, they were transferred to France to join the fighting on the Western Front. They arrived in France in March 1916 and in April became the first Australian battalion to join the battle. In July, 1916, they were at Pozieres, but mainly in carrying duties. In August, they fought at Mouquet Farm, with heavy losses. In 1917, they fought in the second Battle of Bullecourt and then moved to Belgium, they took part in the battle of Broodseinde. After this, they were withdrawn from the line for rest, then took part in defeating the German Spring Offensive in April 1918. With the allies in the ascendancy, the 21st participated in the Hundred Days Offensive, and fought at Hamel, Amiens and Mont St. Quentin. 

Along with the other reinforcements to the 21st Battalion, William Mansfield embarked from Melbourne on 25 September, 1916, on board HMAT A9 Shropshire, arriving at Folkestone, England, on 11 November, 1916.  On 13 December, he was sent to France on board HMAT Princess Henrietta,  marching into Etaples the next day and joining the 21st Battalion at the front on 17 December 1916. That winter was bitter, and the men in the trenches must have suffered badly. William Mansfield, like many others, came down with Trench Foot, caused by constant wet feet, resulting in itching, then swelling, and when, dry, loss of skin on the feet. He was sent back to England for hospitalisation on 21 January, and on 26 January it was decided to return him to Australia as medically unfit,, and in addition ot the porblems with his feet, suffering from senility. He was by then, 58 years old. He returned to Melbourne on board the HT Runic and on 25 August, 1917, was discharged from further military service.

At his age and with his poor state of health, Willam Mansfield may have found it hard to get employment. He re-applied to join the military service and on 17 October, 1917, was  appointed to join the Third District Guard, A Company, Domain, Melbourne. He served until the 26 November, 1918.

From here, he disappears, until 1935, when he applied for his military discharge certificate to be replaced, as it had been stolen. He was living at 27 Young St., Redfern, Sydney. He was still there in 1939, where he died on 13 January. His medals were returned to the military authorities, as he had no will or next of kin.