Norman Sinclair Duncan was twenty years and six months when he joned u, with his father's permission, to serve in the AIF. He was an early recruit, signing up almost immediately war was declared, on 19 August, 1914, and receiving the number 655. he was single, of no trade or calling, but had has three years training with the Cadets prior to enlisting. Physically, he was tall for those days, 5' 11", with brown hair and blue eyes.  His mother may have died fairly recently, her name appears on his emlistment form, but is crossed out and replaced with that of his father, John S. Duncan, living at the Rifle Brigade Hotel in Carlton. 

After training, Norman Duncan was attached to the 6th Battalion. This battalion was formed within two months of the declaration of war. Drawn solely from Victoria, it, along with the 5th, 7th and 8th Battalions, made up the Second Brigade. Two months later, after training, they left for Egypt, landing on 2 December 1914 and marching in to Mena Camp. From there, the 6th battalion went to Gallipoli, landing at Anzac Cove as part of the second wave. ten days later, they took part in the reckless and futile attack on Krithia, which require sprinting over open ground in full view of the enemy. Not surprisingly, one third of the battalion were casualties, including Norman Duncan. He was wounded in the side, so seriously that he was sent back to Australia per the ship Ballarat and declared medically unfit for further duty.

This was not the end of his wartime service. Norman Sinclair re-enlisted on 6 June 1916. His medical examination noted the scars he had accumulated at Krithia: left abdomen; left thigh; right back; under right breast; left back; right leg. Declaring himself to be 23 years and 11 months, a bit surprising given his earlier enlistment form, he was given a new number (2297) and placed in the 39th Battalion.

The 39th Battalion was formed on 27 February 1916 at the Ballarat Showgrounds. After training, they sailed to Britain on 27 May arrriving on 18 July. Following four months further training, they moved into the trenches of the Western Front on 9 December, just in time for the terrible winter of 1916-1917. They first saw action at Messines on 7-9 June, then fought at Broodseinde (4 Oct.), Passchendaele (12 Oct.) and fought at Amiens against the German Spring Offensive. They last saw action at St Quentin Canal in 1918 and were disbanded in March 1919. Norman Duncan, however, missed much of the worst of the war. For much of 1917, he was in hospital, suffering variously from myalgia, pneumonia, high temperatures and vomiting, ear infection. After returning to training, he was court-martialled on 7 July 1917  for accepting money to exclude a soldier from Parade and sentenced to six months jail and forfeiture of wages. After serving his time, he was returned, not to the 39th Battalion, but on 8 May 1918  to the motor transport section. He committed a second offence and on 9 October 1918 was committed to H.M. Prison Shepton, and released on 13 March 1919.

Norman Sinclair returned to Australia, but was to re-enlist in 1939, serving as a pay officer at Lancefield and Royal Parade. In 1922, he married Marie Geneva Contessa, listed in the Electoral Rolls as Mary. In the same year, he is listed as working as a barman at 362 Elizabeth St., Melbourne, where his father is listed as manager. In 1927, he is still a barman, giving as his home address 426 Lygon St., Carlton, with Mary as 'home duties'. By 1942, they are living at Waubra, near Learmonth, Ballarat. He is a labourer, she 'home duties'. According to a letter he wrote to the Defence Department later, they had two sons. From 1949-1954, they are living at 18 Johnston St., Ballarat; Norman lists himself as a labourer, Mary, 'home duties'. 

In 1955, Norman Duncan describes himslef as a T.P.I. pensioner (totally or partially disabled). He died the following year.