Towards the end of August, shortly after the
British entry into World War I, he became GOC of the
9th (Scottish) Division, a newly created
Kitchener's Army formation. This only lasted until October when he was selected to succeed Major General
Hubert Hamilton, GOC of the
3rd Division of the
British Expeditionary Force (BEF) on the
Western Front, who had recently been killed in action. However, he only lasted for a mere two weeks in this post before he was relieved of his command following the inconclusive result at the
Battle of La Bassée later in October. The division departed for service on the Western Front in May and was engaged in the disastrous diversionary battle for the
Somme offensive at
Fromelles on 19 July. This controversial operation, half British and half Australian, led to the loss of many Australian and British soldiers and achieved very little. Casualties were: the
5th Australian Division had 5,513 casualties while Mackenzie's 61st Division had 1,547 casualties. Mackenzie himself continued to serve as the 61st's GOC throughout 1917 and into 1918 until he was wounded by an enemy sniper on 27 April 1918 while he was visiting the line of the
183rd Infantry Brigade south of
St. Floris, being shot through the cheek and
parotid gland. The wound did not respond to treatment and he was evacuated sick to England on 31 May, which marked the end of his active service overseas. He was appointed a
Knight Commander of the Order of the Bath on 31 May. ==Postwar and final years==