In 1929, Tanner had been commissioned in the Militia. In November 1941, Tanner was appointed temporary captain in the 2nd Australian Imperial Force. He was a member of the Gull Force. In January 1942, he became prisoner of war after Gull Force was overpowered by the Japanese in Ambon. Whilst a prisoner of war at Hainan, he organised sports events including a running race between Australian and Dutch prisoners.