In the early days of the First World War he led his regiment, with two squadrons of the 4th Dragoon Guards, in a charge at Elouges on 24 August 1914, as part of the Battle of Mons. He later participated in the final "lance on lance" action of the war on 7 September 1914 at Moncel when he led a charge of two troops of B Squadron of his regiment and overthrew a squadron of the 1st Guard Dragoons. In this second charge, Campbell was wounded several times; the medical officer of the 4th Dragoon Guards, Captain Arthur Osburn, found him sprawled in a field of clover and treated him for "a revolver wound in his leg, a lance wound in his shoulder, and a sword wound in his arm". Despite this, Campbell told the doctor "I've just had the best quarter of an hour I've ever had in my life!" In November 1914 Campbell was promoted to command 6th Cavalry Brigade, then in May 1916 came further promotion to command 21st Division. He retained that command for the rest of the war. His divisional staff gave him the further nickname of "Barbara", the reason for which is no longer known.