Reddish was commissioned into the Second Australian Imperial Force with the service number NX105 on 26 March 1940 and returned to the Citizens Military Force on 15 October 1945. As a battery commander in the &lt;a href="2/6th%20Field%20Regiment%20%28Australia%29"&gt;2/6th Field Regiment&lt;/a&gt;, 7th Division he served in Palestine, Egypt, Mersa-Matruh and Syria. Reddish was awarded the Distinguished Service Order (DSO) for leadership during the Syria–Lebanon campaign. As lieutenant colonel he led his regiment against the Japanese in the New Guinea campaign, fighting in the Owen Stanley Ranges. He also took part in the Salamaua–Lae and Finschhafen actions. Along with the DSO, he was twice Mentioned in Despatches. Reddish was promoted to temporary lieutenant colonel on 15 April 1942 (made substantive on 1 September) and to colonel (temporary brigadier) on 17 November 1943. During his war service he commanded the &lt;a href="2/4th%20Field%20Regiment%20%28Australia%29"&gt;2/4th&lt;/a&gt; and the &lt;a href="2/6th%20Field%20Regiment%20%28Australia%29"&gt;2/6th Field Regiments&lt;/a&gt;. He rose to become Commander Royal Artillery of the 6th Division.