Like the 4th and 5th Devons, the 6th were formed from Volunteer units when the Territorial Army was created in 1908. Based on Barnstaple, the 6th Devons recruited men from the north of the county. With the 4th and 5th Battalions, the 6th were at their annual camp near Exeter when the First World War began. Embodied on the outbreak of war, the Battalion assembled on Salisbury Plain for training and to receive reinforcements. A second battalion – known initially as the 6th (Reserve) Battalion but soon retitled the 2/6th – was formed from Territorials who had not elected to serve overseas and from the recruits who flooded in after war had been declared.
Meanwhile, the 1/6th had embarked for India on 9th October 1914 and were stationed near Lahore and engaged in internal security operations. The 2/6th followed in January 1915 and were stationed at Bombay, then Muttra and finally Peshawur. During their long period in India they sent drafts of trained men to other units serving in Mesopotamia, Egypt and Palestine.
At the end of 1915 the 1/6th were sent to Mesopotamia and they arrived at Basra on 3rd January 1916. Marching 300 miles up country through swamps, marshes and deserts, they suffered sickness and attacks from local Arabs before reaching Orah. From here, in early March, they launched an attempt to relieve the besieged garrison at Kut el Amara. Tantalisingly, they were within sight of Kut when a bungled artillery bombardment warned the Turks of their advance and heavy shell and machine gun fire forced them to retreat. The 1/6th lost 201 casualties and were congratulated on their bravery. In April they played a supporting role in the battle of Sannaiyat and in May launched an attack on the Dujullah Redoubt. Blazing heat and lack of water caused more casualties and, despite drafts of 180 men from the 2/5th and 2/6th, in July their total strength was 7 officers and 180 men.
The years 1917 and 1918 saw the 1/6th on the Twin Canals, where they relieved the 1/4th Devons, and on lines of communication duties in a harsh environment which continued to exact a high price in sickness. In September 1918 at Margil they were struck by influenza, which alone claimed 89 lives. During their three years in Mesopotamia the 1/6th lost twice as many men to illness as to enemy action. They had won three battle honours.
In September 1917 the 2/6th Devons docked in Basra from India and spent the rest of the war on lines of communication duties near Sheikh Saad. Although they had seen no action as a battalion, they had supplied their sister battalions with a great many trained soldiers.