On 1 August 1914 - three days before Britain declared war - the Governor-General of Canada offered help should war break out. On 7 August Lord Kitchener responded that the Canadians might form a Division. An Expeditionary Force soon began to assemble at Valcartier (Quebec). The Division sailed on 3 October 1914 and arrived in England eleven days later. Training re-commenced on Salisbury Plain and the Division embarked for St Nazaire in early February 1915. It arrived in the Hazebrouck-Strazeele area on 15 February and thereafter the Division fought in many of the major actions of the war, including:

1915
The Second Battle of Ypres
The Battle of Festubert
The Second Action of Givenchy

1916
The Battle of Mount Sorrel
The Battle of Flers-Courcelette*
The Battle of Thiepval*
The Battle of Le Transloy*
The Battle of the Ancre Heights*
The battles marked * are phases of the Battles of the Somme 1916

1917
The Battle of Vimy**
The Battle of Arleux**
The Third Battle of the Scarpe and the capture of Fresnoy**
Operations towards Lens**
The battles marked ** are phases of the Arras Offensive
The Second Battle of Passchendaele (a phase of the Third Battle of Ypres)

1918
The Canadian 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. They were also fortunate in avoiding the defensive fighting against the German offensives of spring 1918.
The Battle of Amiens and actions around Damery
The Battle of the Scarpe+
The Battle of Drocourt-Queant+
The battles marked + are phases of the Second Battles of Arras 1918
The Battle of the Canal du Nord++
The Battle of Cambrai++
The battles marked ++ are phases of the Battles of the Hindenburg Line

The Division was selected to advance into Germany but demobilisation commenced in late 1918.

The order of battle of the 1st Canadian Division

1st Canadian Brigade
1st Bn, the Canadian Infantry (Western Ontario)
2nd Bn, the Canadian Infantry (Eastern Ontario)
3rd Bn, the Canadian Infantry (Toronto)
4th Bn, the Canadian Infantry (Central Ontario)
1st Canadian Machine Gun Company	formed January 1916 left to move into 1st Canadian MG Battalion March 1918
1st Canadian Trench Mortar Battery	formed May 1916
2nd Canadian Brigade
5th Bn, the Canadian Infantry (Western Cavalry)
6th Bn, the Canadian Infantry (Fort Garry Horse)	left December 1914
7th Bn, the Canadian Infantry (1st British Columbia)
8th Bn, the Canadian Infantry (90th Rifles)
10th Bn, the Canadian Infantry (Canadian)	joined December 1914
2nd Canadian Machine Gun Company	formed February 1916 left to move into 1st Canadian MG Battalion March 1918
2nd Canadian Trench Mortar Battery	formed April 1916
3rd Canadian Brigade
13th Bn, the Canadian Infantry (Royal Highlanders)
14th Bn, the Canadian Infantry (Royal Montreal)
15th Bn, the Canadian Infantry (48th Highlanders)
16th Bn, the Canadian Infantry (Canadian Scottish)
3rd Canadian Machine Gun Company	formed March 1916 left to move into 1st Canadian MG Battalion February 1918
3rd Canadian Trench Mortar Battery	formed April 1916
4th Canadian Brigade	broken up by January 1915
9th Bn, the Canadian Infantry	converted into a reserve unit
10th Bn, the Canadian Infantry (Canadian)	to 1st Brigade in December 1914
11th Bn, the Canadian Infantry
12th Bn, the Canadian Infantry
Divisional Troops	under direct command of Divisional HQ
17th Bn, the Canadian Infantry (Nova Scotia Highlanders)	left January 1915, became reserve unit
18th Bn, the Canadian Infantry	disbanded September 1914
Newfoundland Contingent	left December 1914, later formed complete Battalion
1st Canadian Pioneer Battalion	formed march 1916
13th Canadian Machine Gun Company	joined January 1917 left to move into 1st Canadian MG Battalion March 1918
1st Canadian Machine Gun Battalion	formed March 1918
1st Canadian Divisional Train ASC	later titled 437, 438, 439 and 440 Companies
1st Canadian Mobile Veterinary Section AVC
1st Canadian Divisional Employment Company	joined June 1917
Divisional Mounted Troops	under direct command of Divisional HQ
Service Sqn, the 19th Alberta Dragoons	left May 1916
1st Canadian Cyclist Company	left May 1916
Royal Canadian Dragoons	joined October 1914, left February 1915
Strathcona's Horse	joined October 1914, left February 1915
Automobile Machine Gun Brigade No 1	left February 1915
Divisional Artillery
1st Canadian Brigade, RFA
2nd Canadian Brigade, RFA
3rd Canadian Brigade, RFA	left May 1917
Royal Canadian Horse Artillery RHA	joined September 1914, left February 1915
1st Canadian Divisional Ammunition Column
CXVIII (Howitzer) Brigade, RFA	British unit. Joined February 1915, left June 1916
22nd Canadian Brigade, RFA	formed June 1916, disbanded March 1917
W.1.C Heavy Trench Mortar Battery RFA	joined June 1916, retitled as V.1.C in 1918
X.1.C, Y.1.C and Z.1.C Medium Mortar Batteries RFA	formed March 1916; in February 1918, Z broken up and batteries reorganised to have 6 x 6-inch weapons each
Royal Engineers	Field Companies formed into a Brigade in June 1918
1st Canadian Field Company
2nd Canadian Field Company
3rd Canadian Field Company
1st Canadian Divisional Signals Company
Royal Army Medical Corps
1st Canadian Field Ambulance
2nd Canadian Field Ambulance
3rd Canadian Field Ambulance