 Illingworth saw action in France in 1918, which ended when he was shot down (forced landing) in August over the Somme and held as a prisoner-of-war at Ingolstadt, Bavaria, Germany.After returning home following the end of the war, he continued his studies at Edinburgh and graduated MB ChB (Bachelor of Medicine, Bachelor of Surgery) in 1922.After qualifying as a medical doctor, Illingworth undertook a period as house physician at the Derbyshire Royal Infirmary.