Shoulder flash King's Own Yorkshire Light Infantry (K.O.Y.L.I.)

During the Second World War the battalions served in all three fronts (Europe, North Africa and Asia-Pacific).

The 1st Battalion, a Regular Army unit, was serving with the 15th Infantry Brigade, alongside the 1st Battalion, Green Howards and the 1st Battalion, York and Lancaster Regiment, and was sent to France in 1939, as part of 5th Infantry Division. In May 1940, with the brigade, the battalion was sent to Norway where they saw service as part of Sickleforce in the Norwegian Campaign and was later evacuated. The battalion next saw action three years later in the Allied invasion of Sicily, and later the Allied invasion of Italy, and fought through the Italian Campaign, in the Battle of Anzio and Operation Diadem before, in July 1944, being sent to Palestine to rest and refit, and the division remained there until early 1945 when it was transferred to Italy again, only to be transferred to North-western Europe for the invasion of Germany, where the battalion ended its war.

The 2nd Battalion, also a Regular Army unit, fought as a rearguard in the retreat through Burma in 1942. However, the battalion saw little active service afterwards, remaining in British India on internal security duties.

The 1/4th Battalion served throughout the entire North West Europe Campaign crossing the English Channel several days after D-Day as part of the 146th Infantry Brigade, 49th (West Riding) Infantry Division, fighting in the Battle of Normandy in the summer of 1944, fighting during the battle for Caen during Operation Martlet, the Second Battle of the Odon and later clearing the Channel Coast in Operation Astonia, garrisoning "The Island" during the aftermath of Operation Market Garden, where they then fought in the Battle of the Scheldt. The battalions' last battle was during the Second Battle of Arnhem.

The 2/4th Battalion was created in 1939 in the Territorial Army when the 4th Battalion was split in two, creating the 1/4th Battalion and the 2/4th Battalion. The 2/4th fought with the 138th Infantry Brigade, part of the 46th Infantry Division in the Battle of Dunkirk with the rest of the BEF. They also fought in the Tunisia Campaign in the final stages of the North African Campaign and later in the Italian Campaign.

The 5th Battalion was transferred to the Royal Artillery before the war and converted into the 53rd Light Anti-Aircraft Regiment, Royal Artillery. From August 1940 until September 1942 the regiment served in Home Forces and joined the 10th Armoured Division, and was sent overseas, joining the British Eighth Army and served in the Italian Campaign for the rest of the war, from September 1943 shortly after the initial invasion.

The 7th Battalion became the 149th Regiment in the Royal Armoured Corps in 1941, but continuing to wear their King's Own Yorkshire Light Infantry cap badge on the black beret of the RAC.

The 8th Battalion was a war service battalion raised in July 1940 and was also transferred to the Royal Artillery and became the 94th Light Anti-Aircraft Regiment, Royal Artillery. In June 1942 the regiment became part of the Guards Armoured Division and served with it for the rest of the war, fighting in North-western Europe from June 1944 until May 1945.

The 9th Battalion (formerly the Queen's Own Yorkshire Dragoons)[19] was motorized infantry assigned to the 18th Infantry Brigade and attached to the 1st Armoured Division. The battalion served in the Italian Campaign, fighting particularly severe battles such as that during the Battle of Anzio in early 1944 until, due to a severe shortage of manpower in the Italian theatre, it was disbanded later in the year.