Born in/at New Plymouth, Taranaki, Manu began his rugby career with Waikato, the province immediately to the north of Taranaki. He made his debut for Waikato in 1999, playing against a Japanese XV. He made a further 65 appearances for Waikato during his time there. In 2001, he was selected for the Waikato Chiefs Super 12 squad. He made his debut for the Chiefs in their opening game of the 2001 Super 12 season, a 42–23 loss to the Waratahs. Manu played for the Chiefs for a further five years before being signed by the Welsh club Scarlets in the summer of 2006. He made his competitive debut for the Scarlets on 8 September 2006, coming on as a 29th-minute substitute for John Davies in a 31–17 victory over the Glasgow Warriors.
In the lead-up to the 2007 Rugby World Cup, Manu expressed his availability for selection to the Fijian selectors. Although he had played all of his representative rugby for New Zealand teams up to that point, Manu was eligible for Fiji through his Fijian mother. Manu made his debut for Fiji in 2009 against Scotland and captained Fiji in their game against Australia in June 2010. He later returned for the 2010 end of year tour and was handed the captaincy of the Fiji team again. He later captained the side in the 2011 Rugby World Cup. He last represented Fiji on 17 November 2012 against the Ireland Wolfhounds.
On 28 April 2014, Manu announced that he would retire at the end of the 2013-14 European season, and take up the role of Head Coach for Hong Kong Cricket Club, which is a rugby union club that plays in the HKRFU Premiership.