On 3 August 1914 - the day before Britain declared war - the Government of Australia offered an expeditionary force of 20,000 men, to be composed of a Division plus a brigade of Light Horse and titled the Australian Imperial Force. The original AIF sailed on 1 November 1914 and proceeded via Colombo to Egypt. The original intention was that the force would continue on to England but problems of accommodation being experienced by the Canadian units then in England forced a change of mind. The Force halted in Egypt and training recommenced. Thereafter the Division fought in many of the major actions of the war, including:

1915
3 February 1915: two battalions involved in defending against Turkish attack on Suez Canal
March 1915 : 3rd Brigade moves to Lemnos
1 April 1915 : Division receives orders to prepare to make amphibious assault at Gallipoli
25 April 1915 : Division lands at what was to become forever known as ANZAC Beach
The Division withdrew from Gallipoli in late 1915.

1916
On 13 January 1916 it was decided that the AIF would be developed to create five Divisions, four Australian and one New Zealand. The infantry brigades of 1st Division were split to create a nucleus of the 14th, 15th and 16th Brigades and places in all six brigades were filled by new drafts. The Division was warned for France and sailed on 13 March 1916. It remained in France and Flanders for the rest of the war:
The Battle of Pozieres including the fighting for Mouquet Farm*
The battle marked * is a phase of the Battles of the Somme 1916

1917
Operations on the Ancre including the capture of the Thilloys
The Arras Offensive, in which the Division fought the German attack on Lagnicourt and in the Battle of Bullecourt
The Battle of the Menin Road**
The Battle of Polygon Wood**
The Battle of Broodseinde**
The battle of Poelcapelle**
The Second Battle of Passchendaele**
The battles marked ** are phases of the Third Battle of Ypres

1918
The Australian Divisions were not affected by the restructuring of the army to having three brigades per Division, that took place in the British Divisions in February 1918. When the enemy launched its second phase of the spring offensive, the Division was rushed from the Somme to help plug the gap:
The Battle of Hazebrouck in which the Division fought the defence of Nieppe Forest***
The battle marked *** is a phase of the Battles of the Lys
The Battle of Amiens
The Battle of Albert in which the Division captured Chuignes+
The battle marked + is a phase of the Second Battles of the Somme1918
The Battle of Epehy++
The battle marked ++ is a phase of the Battles of the Hindenburg Line

The Division was in Australian Corps reserve at the Armistice and was not selected to advance into Germany. Demobilisation commenced in early 1919 and by March 1919 the Division merged with 4th Australian Division.

The order of battle of the 1st Australian Division

1st Australian Brigade
1st Bn, the Australian Infantry
2nd Bn, the Australian Infantry
3rd Bn, the Australian Infantry
4th Bn, the Australian Infantry
1st Australian Machine Gun Company	formed February 1916 left to move into 1st Australian MG Battalion February 1918
1st Australian Trench Mortar Battery	formed April 1916
2nd Australian Brigade
5th Bn, the Australian Infantry
6th Bn, the Australian Infantry
7th Bn, the Australian Infantry
8th Bn, the Australian Infantry
2nd Australian Machine Gun Company	formed February 1916 left to move into 1st Australian MG Battalion February 1918
2nd Australian Trench Mortar Battery	formed April 1916
3rd Australian Brigade
9th Bn, the Australian Infantry
10th Bn, the Australian Infantry
11th Bn, the Australian Infantry
12th Bn, the Australian Infantry
3rd Australian Machine Gun Company	formed February 1916 left to move into 1st Australian MG Battalion February 1918
3rd Australian Trench Mortar Battery	formed April 1916
Divisional Troops	under direct command of Divisional HQ
1st Australian Pioneer Bn	formed in Egypt
21st Australian Machine Gun Company	joined February 1917 left to move into 1st Australian MG Battalion February 1918
1st Australian Machine Gun Battalion	formed February 1918
1st Australian Divisional Train ASC	later titled 741, 742, 743 and 744 Companies
1st Australian Mobile Veterinary Section AVC
1st Australian Divisional Employment Company	joined August 1917
Divisional Mounted Troops	under direct command of Divisional HQ
4th Australian Light Horse	left April 1915, returned after Gallipoli and left March 1916
1st Australian Cyclist Company	formed in Egypt, left May 1916
Divisional Artillery
1 Australian Brigade, RFA
2 Australian Brigade, RFA
3 Australian Brigade, RFA
21 Australian (Howitzer) Brigade, RFA	formed in Egypt, absorbed by July 1916
1st Australian Divisional Ammunition Column
1st Australian Heavy Battery RGA	formed at Gallipoli in November 1915, disbanded in December
V.1.A Heavy Trench Mortar Battery RFA	formed June 1916, left February 1918
X.1.A, Y.1.A and Z.1.A Medium Mortar Batteries RFA	formed April 1916; in February 1918, Z broken up and batteries reorganised to have 6 x 6-inch weapons each
Royal Engineers
1st Australian Field Company
2nd Australian Field Company
3rd Australian Field Company
1st Australian Divisional Signals Company
Royal Army Medical Corps
1st Australian Field Ambulance
2nd Australian Field Ambulance
3rd Australian Field Ambulance