Per his military service record (regimental no. 4767), Frank Duncan was born at Bourke, N.S.W. He gave his age as 22 years and 3 months, his marital status as single, and his occupation as labourer. His description on his Certificate of Medical Examination was height 5 feet 10 inches tall, weight 154 lbs., with a dark complexion, brown eyes, and black hair. His religious denomination was Church of England. He claimed to have had no previous military service.
Frank Duncan completed his medical examination, along with several other men from Cobar, at Dubbo on 2nd November 1915, (the day the Coo-ees were at Lithgow). He was attested at Dubbo on 7th November 1915. It appears he then caught up with the other Cobar boys (including Walter and Robert Mitchell, Andrew Lennox and Norman Francisco), to join the Coo-ee March somewhere in the Blue Mountains.
After completing the Coo-ee March he went to Liverpool Camp as reinforcement for the 13th Battalion.
On 13th January 1916 Private Duncan was charged with Being Absent Without Leave (1 day).from Liverpool Camp. He was fined 5 Shillings. On 7th February 1916 he was charged with Being Absent from Piquet. He was fined 5 Shillings.
A farewell was held at the Masonic Hall in Cobar on Friday 3rd March 1916 to bid farewell to Cobar boys Private Duncan and brothers Private Bob Mitchell and Corporal Walter Mitchell, when they were on final leave before departing overseas. 
On his embarkation roll his address at time of enrolment was Monaghan Street, West Cobar, N.S.W., and his next of kin was listed as his father, F. Duncan, at the same address.
On 8th March 1916 Private Duncan, along with many of the other Coo-ees, departed Sydney on the HMAT A15 Star of England, with the 15th reinforcements for the 13th Battalion.
He arrived in Egypt on 11th April 1916.
On 16th April 1916 Private Duncan was transferred to the 4th Pioneer Battalion at Telelkebir.
On 4th June 1916 Private Duncan left Alexandria aboard the Transport Scotian bound for France. He arrived at Marseilles on 11th June 1916. He was then sent to the 4th Australian Division Base Depot at Etaples, France.
On 27th August 1916 Private Duncan was sent to hospital sick, and on 28th August 1916 he was admitted to the 11th Stationary Hospital at Rouen, France, suffering a concussion of the brain. (A later memo entry in his service record notes that ‘This soldier whilst asleep fell on to a stone floor from top bunk (bunks three tiers high) soldier not to blame’, which appears to relate to this injury). On1st September 1916 he was placed aboard the Hospital Ship Salta at Le Havre. On 2nd September 1916 he was admitted to the Meath Hospital at Dublin in Ireland. On 9th October 1916 he was transferred to the Holywood Military Hospital in Ireland. On 24th October 1916 he was transferred to the 1st Australian Auxiliary Hospital at Harefield, England. On 31st October 1916 he was discharged from Hospital and sent to the No. 2 Command Depot at Weymouth, England.
On 4th November 1916 Private Duncan was granted leave to report to the No.4 Command Depot at Wareham, England on 18th November 1916, where he marched in from furlough on 20th November 1916.
On 22nd January 1917 Private Duncan was charged with Being Absent Without Leave from 9.30 pm on 14th January 1917 till 2.30 pm on 19th January 1917. He was awarded 144 hours detention and forfeited 12 days pay.
On 29th March 1917 Private Duncan was transferred to the No.3 Command Depot at Hurdcott, England.
On 11th April 1917 Private Duncan was charged with Failing to obey Routine Orders by being out of bounds and of Being Absent Without Leave from 7 am on 7th April 1917 till 11.30 pm on 7th April 1917. He was awarded 6 days confined to camp.
On 14th April 1917 Private Duncan was transferred from Hurdcott to the Australian H. & D. Depot at Perham Downs. On the 21st of April 1917 he was transferred to the Pioneer Training Battalion at the No. 4 Camp at Perham Downs, England.
On 26th May 1917 Private Duncan was transferred to the No. 1 Command Depot at Perham Downs, England.
On 13th June 1917 Private Duncan was charged with Being Absent Without Leave from 12 noon on 11th June 1917 till 3 pm on 12th June 1917. He was admonished and forfeited 2 days pay.
On 16th July 1917 Private Duncan was detached for duty with the Headquarters of the AIF Depots in the United Kingdom. He returned to the No. 4 Camp at Perham Downs on 3rd August 1917.
On 12th February 1918 Private Duncan departed Southampton, England bound for France. He arrived at the 4th Australian Division Base Depot at Le Harve, France on 13th February 1918.
On 15th February 1918 Private Duncan was charged with Being Absent Without Leave from 10 pm on 13th February 1918 till 10 pm on 14th February 1918. He was awarded 16 days field punishment no. 2 and forfeited 18 days pay.
Private Duncan re-joined the 4th Pioneer Battalion in France on 20th February 1918.
On 1st March 1918 Private Duncan was detached from the 4th Pioneer Battalion for duty with the 184th Tunnelling Company. He re-joined the 4th Pioneer Battalion on 24th March 1918.
On 12th September 1918 Private Duncan was sent to the 12th Australian Field Ambulance sick. On 13th August 1918 he was moved to the 41st Stationary Hospital. On 16th September 1918 he was transferred to the 1st Stationary Hospital at Rouen. On 18th September 1918 he was transferred to the 39th General Hospital at Le Harve, France. On 12th November 1918 he was discharged and sent to the 4th Australian Division Base Depot. He departed the 4th Australian Division Base Depot on 17th November 1918 and re-joined the 4th Pioneer Battalion on 19th November 1918.
On 27th January 1919 Private Duncan marched into the Australian Division Base Depot at Le Harve, France to commence his return to Australia. On 10th February 2019 he departed Le Havre, for England. He arrived at Weymouth, England on the 11th of February 1919 and marched into the 3rd Training Brigade.
On 16th March 1919 Private Duncan was sent to Hospital suffering from a Septic Hand. On 19th March 1919 he was transferred to the 1st Australian General Hospital at Sutton Veny. He was discharged on 31st March 1919 and returned to the 3rd Training Brigade.
On 13th April 1919 Private Duncan departed England on the Transport Commonwealth, bound for Australia.
He arrived in Sydney on 12th June 1919, and was discharged Medically Unfit on 18th September 1919.
 NAA: B2455, DUNCAN F.
 Corp. E. J. Hitchins [sic], Pte F. Duncan. (1916, October 25). Sydney Mail (NSW : 1912 – 1938), p. 26. Retrieved September 17, 2022, from http://nla.gov.au/nla.news-article160388889 ; Kay Stingemore, From the Far West to the Western Front : Cobar and the Great War, Cobar : Great Cobar Heritage Centre, c2015, p. 192.
 Cobar’s Farewell. To Corporal Mitchell, Privates Mitchell and Duncan. (1916, March 10). Western Age (Dubbo, NSW : 1914 – 1932), p. 2. Retrieved September 17, 2022, from http://nla.gov.au/nla.news-article136721446
 Australian War Memorial. First World War Embarkation Rolls, Fred Duncan, HMAT Star of England A15, 8th March 1916.
 NAA: B2455, DUNCAN F., Casualty Form – Active Service Regimental No. 4767, Pte. Frank Duncan, entry for Memo from C.O. 4th Pioneer Btn AIF, France, to 3rd Echelon G.H.Q. 25 January 1917.