Born in Ohinemutu, New Zealand, Manahi worked as a labourer when, in November 1939, he volunteered to join the Māori Battalion, newly raised for service in the Second World War. In 1941, he participated in the Battle of Greece and fought in the Battle of Crete during which he was wounded. After recovering from his wounds, he returned to his unit and fought through the Western Desert and Tunisian campaigns, during which he was recommended for a VC for his actions at Takrouna over the period 19–21 April 1943. Despite the support of four generals, his VC nomination was downgraded to an award of a DCM, possibly by the British Chief of the General Staff, General Alan Brooke.