Callinan was commissioned as a lieutenant in the Citizens Military Force in June 1940, being appointed into the Royal Australian Engineers. In March 1941, he volunteered for overseas service and joined the Second Australian Imperial Force, taking up a posting as an instructor at the Demolitions Wing, at the 7th Infantry Training Centre. He was promoted to captain in July 1941, and subsequently posted as the second-in-command to the &lt;a href="2/2nd%20Commando%20Squadron%20%28Australia%29"&gt;2/2nd Independent Company&lt;/a&gt;, deploying with them to Timor as part of Sparrow Force in December 1941. He subsequently led the commandos during their campaign on the island and in July 1942 was temporarily promoted to major.