In October 1939, Mills was appointed to the Second Australian Imperial Force and served with the 6th Divisional Reconnaissance Cavalry Regiment. At the Battle of Bardia in January 1941 he commanded three troops of Bren-gun carriers and was awarded the Military Cross for bravery in the field. During the invasion of Syria in June that year he was awarded a Bar to his Military Cross. Mills was the first of only fifteen Australian soldiers to hold that distinction in World War II. He was promoted to captain in June 1941 and major in February 1942 and in March 1943 was promoted to temporary lieutenant colonel and made commander of the &lt;a href="2/5th%20Armoured%20Regiment%20%28Australia%29"&gt;2/5th Armoured Regiment&lt;/a&gt;. In May 1944, Mills took command of the &lt;a href="2/4th%20Armoured%20Regiment%20%28Australia%29"&gt;2/4th Armoured Regiment&lt;/a&gt; and served in Madang, New Guinea. He relinquished command of the regiment in February 1946 and transferred to the Reserve of Officers on 27 March.