A Division of K3, the units were assembled around Tring during September 1914. The Division crossed to France 2nd-13th September 1915. It served on the Western Front with distinction throughout the war.

The Division's first experience was truly appalling. Having been in France for only a few days, lengthy forced marches brought it into the reserve for the British assault at Loos. GHQ planning left it too far behind to be a useful reinforcement on the first day, but it was sent into action on 26th September, whereupon it suffered over 3,800 casualties for very little gain.

The Battle of Loos

The Battle of Albert (first phase of the Battle of the Somme 1916)

The Battle of Bazentin (second phase of the Battle of the Somme 1916)

The Battle of Flers-Courcelette (sixth phase of the Battle of the Somme 1916)

The Battle of Morval (seventh phase of the Battle of the Somme 1916)

In the above action, the Division captured Gueudecourt

The Battle of Le Transloy (eighth phase of the Battle of the Somme 1916)

The German Retreat to the Hindenburg Line

The First Battle of the Scarpe (first phase of the Arras Offensive)

The Third Battle of the Scarpe (fourth phase of the Arras Offensive)

Flanking Operations Round Bullecourt

The Battle of the Polygon Wood (fourth phase of the Third Battle of Ypres)

The Battle of Broodseinde (fifth phase of the Third Battle of Ypres)

The Second Battle of Passchendaele (eighth phase of the Third Battle of Ypres)

The Battle of Cambrai

The Battle of St Quentin (first phase of the First Battles of the Somme 1918)

The First Battle of Bapaume (second phase of the First Battles of the Somme 1918)

The Battle of Messines, 1918 (second phase of the Battles of the Lys) (62nd Brigade)

The Second Battle of Kemmel (seventh phase of the Battles of the Lys)

The Battle of the Aisne 1918

The Battle of Albert (first phase of the Second Battles of the Somme 1918)

The Battle of Bapaume (second phase of the Second Battles of the Somme 1918)

The Battle of Epehy (second phase of the Battles of the Hindenburg Line)

The Battle of the St Quentin Canal (fourth phase of the Battles of the Hindenburg Line)

The Battle of Cambrai 1918 (sixth phase of the Battles of the Hindenburg Line)

The Battle of the Selle

The Division ceased to exist on 19th May 1919, having suffered casualties of more than 55,000 during the war.

62nd Brigade

12th (Service) Bn , the Northumberland Fusiliers (joined September 1914)

13th (Service) Bn , the Northumberland Fusiliers (joined September 1914)

3/4th Bn (TF), the Queen's (joined 9 August 1917)

1st Bn, the Lincolns (joined November 1915)

2nd Bn, the Lincolns (joined February 1918)

8th (Service) Bn, the East Yorkshires(joined September 1914, left November 1915)

10th (Service) Bn, the Yorkshire (joined October 1914,disbanded February 1918)

62nd Brigade Machine Gun Company (joined on 4 March 1916, moved into 21 MG Bn 24 February 1918)

62nd Trench Mortar Battery (formed by 16 June 1916)

63rd Brigade

8 July 1916 : the Brigade was transferred to 37th Division

8th (Service) Bn, the Lincolns (joined September 1914)

8th (Service) Bn, the Somerset Light Infantry (joined October 1914)

12th (Service) Bn, the West Yorkshires(joined September 1914, left November 1915)

4th Bn, the Middlesex (joined November 1915, left July 1916)

10th (Service) Bn, the York and Lancaster (joined October 1914, left July 1916)

63rd Brigade Machine Gun Company (joined on 4 March 1916, left July 1916)

63rd Trench Mortar Battery (formed by 16 June 1916, left July 1916)

64th Brigade

1st Bn, the East Yorkshires(joined November 1915)

2nd Bn, the South Lancashire (joined June 1918, left same month)

9th (Service) Bn, the KOYLI (joined October 1914)

10th (Service) Bn, the KOYLI (joined October 1914, disbanded February 1918)

14th (Service) Bn, the Durham Light Infantry (joined September 1914, left November 1915)

15th (Service) Bn, the Durham Light Infantry (joined September 1914)

64th Brigade Machine Gun Company (joined on 4 March 1916, moved into 21 MG Bn 24 February 1918)

64th Trench Mortar Battery (formed by 19 June 1916)

110th Brigade

7 July 1916 : Brigade transferred from 37th Division

6th (Service) Bn, the Leicesters

7th (Service) Bn, the Leicesters

8th (Service) Bn, the Leicesters (left June 1918)

9th (Service) Bn, the Leicesters (disbanded February 1918)

1st Bn, the Wiltshires (joined June 1918)

110th Brigade Machine Gun Company (joined with Brigade, moved into 21 MG Bn 24 February 1918)

110th Trench Mortar Battery (joined with Brigade)

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Divisional Troops

14th (Service) Bn , the Northumberland Fusiliers (joined September 1914, became Pioneers January 1915)

13th (Service) Bn, the King's Royal Rifle Corps (joined October 1914, left April 1915)

13th (Service) Bn, the Rifle Brigade (joined October 1914, left April 1915)

237th Machine Gun Company (joined 17 July 1917, moved into 21 MGBn 24 February 1918)

No 21 Machine Gun Battalion (created 24 February 1918)

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Divisional Mounted Troops

A Squadron , the South Irish Horse (joined January 1915, left May 1916)

21st Divisional Cyclist Company (formed in February 1915, left 10 May 1916)

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Divisional Artillery

XCIV Brigade, RFA

XCV Brigade, RFA

XCVI Brigade, RFA (broken up 13 January 1917)

XCVII (H) Brigade, RFA (broken up 28-31 August 1916)

21st Heavy Battery, RGA (a Battery of 4 60-lbr guns raised with the Division but moved to XXII Heavy Artillery Brigade on formation)

21st Divisional Ammunition Column

V.21 Heavy Trench Mortar Battery RFA (joined 1 June 1916, transferred to Corps Troops February 1918)

X.21, Y.21 and Z.21 Medium Mortar Batteries RFA (joined by 1-13 March 1916; in February 1918, Z broken up and batteries reorganised to have 6 x 6-inch weapons each)

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Engineer Units

85th Field Company (joined Sep 1914, left Jan 1915)

86th Field Company (joined Sep 1914, left Feb 1915)

97th Field Company (joined Jan 1915)

98th Field Company (joined Jan 1915)

126th Field Company (joined Mar 1915)

94th Field Company (joined Jan 15)

21st Divisional Signal Company (joined Sep 1914)

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Field Ambulances

63rd Field Ambulance (joined Oct 1914)

64th Field Ambulance (joined Oct 1914)

65th Field Ambulance (joined Oct 1914)

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21st Divisional Train ASC (182, 183, 184 and 185 Coys ASC joined November 1914)

222nd Divisional Employment Company (joined 30 June 1917)

33rd Mobile Veterinary Section (joined early 1915)

38th Sanitary Section ( left 1 April 1917)

21st Divisional Motor Ambulance Workshop (broken up in April 1916)