Due to the string of defeats during Black Week that same month, the British government realised they were going to need more troops than just the regular army, thus issuing a Royal Warrant on 24 December.

The Royal Warrant asked standing Yeomanry regiments to provide service companies of approximately 115 men each.

[10] The 2nd Line regiment was formed in September 1914 and in January 1915 was assigned to the 2/1st Highland Mounted Brigade.

The regiment was disbanded in early 1917 with personnel transferring to the 2nd Line or to the 4th (Reserve) Battalion of the Black Watch at Ripon.

Following the experience of the war, it was decided that only the fourteen most senior yeomanry regiments would be retained as horsed cavalry,[13] with the rest being transferred to other roles.

[14] As a result, on 6 January 1921, the Regiment was one of eight[a] converted and reduced to 2nd (Fife and Forfar) Armoured Car Company, Tank Corps, later renumbered as 20th (Fife and Forfar) Armoured Car Company, Royal Tank Corps.

[6] By 1939, it had become clear that a new European war was likely to break out, and the doubling of the Territorial Army was authorised, with each unit forming a duplicate.

[17] The 1st Fife and Forfar Yeomanry landed in France as reconnaissance regiment for the 51st (Highland) Infantry Division in the British Expeditionary Force in September 1939 and then took part in the Dunkirk evacuation in June 1940.

[23] A book of remembrance is housed at Cupar Old Parish Church[24] and there is a carved stone plaque dedicated to "To the memory of all ranks – The Fife Light Horse and The Fife and Forfar Yeomanry 1860–1918" inside the ruined church building at Tulliallan Old Churchyard (also known as Woodlea Old Cemetery) at Kincardine on Forth.

[25] The battle honours of the Fife and Forfar Yeomanry are displayed on the guidon which was laid up at Cupar Old Parish Church when the regiment was amalgamated.

The Fife and Forfar Yeomanry was awarded the following battle honours (honours in bold are emblazoned on the regimental colours):[6] The regiment's guidon is laid up in Cupar Old and St Michael of Tarvit Parish Church.

It was designed for the high-speed express passenger services on the East Coast Main Line between London King's Cross and Edinburgh.