In December 1916, Farrell returned to the Front as a war artist, attached for three weeks to the 15th, 16th and 17th Highland Light Infantry in Flanders. In November 1917 he was in France, attached for two months to the staff of the 51st (Highland) Division. In between, authorized by the Minister of Munitions and Admiralty, and supported by Glasgow's Lord Provost, Farrell drew the heroic home effort of women in Glasgow's munitions factories, shipyards and engineering works.
As a former soldier, Farrell's sketches and watercolours of the Front powerfully offer a landscape filtered through personal experience and emotion. Battle scenes and strategic deliberations are reconstructed, informed by first-hand accounts. Many include portraits of actual soldiers. There are poignant images of graves, devastated landscapes and destroyed churches. However, there are also scenes of reconstruction and renewed activity amid the desolation. He is at his most dynamic in his drawings of the munitions factories which are full of noise, light and movement. In these there is a sense of joy and energy in industry and machinery, in patterning and design.
The commission Farrell received from the Corporation of Glasgow to produce 50 drawings of the front line and munitions factories in the city to record the war for posterity was extraordinary. He was unique in being the only war artist to be commissioned by a city rather than by the government, Imperial War Museum or armed forces. Glasgow was one of the first cities to recognize the importance of creating such a memorial, rather than just creating images for propaganda purposes.