Manahi was born in Ohinemutu, New Zealand. He worked as a labourer (jobs that use manual labour). In November 1939, he joined the Māori Battalion. In 1941, he was part of the Battle of Greece and fought in the Battle of Crete. He was wounded in the Battle of Crete. After he was better, he fought in the Western Desert and the Tunisian campaign. Some people wanted him to get a VC because of his work at Takrouna. However, he was given a DCM in April 1943.