Born in Auckland, New Zealand to Samoan parents, Matagi is the second youngest of six brothers. He attended Kelston Boys' High School before dropping out at age 14, having become involved in gang violence and heavy alcohol abuse. In 2006, at the age of 17, Matagi was sentenced to three years in prison for assaulting two boys during a drunken rage that occurred when he was 15. He served his sentence at Mount Eden Prison and later Manawatu Prison, before his participation in rehabilitation courses saw him granted parole after one year.
Following his release from prison, Matagi took up rugby league in 2008, having never previously played any organised sport. He played for the Te Atatu Roosters and later the Mount Albert Lions in the Auckland Rugby League competition. Matagi was selected to play for the Auckland representative team, and the New Zealand Residents for four consecutive years between 2009 and 2012. He was named man of the match in Auckland's 44-34 win over the South Island Scorpions in the National Competition's 2011 Grand Final.
Matagi made his debut for the Auckland Vulcans, the New Zealand Warriors' feeder team, in the New South Wales Cup on 3 April 2010. He played regularly for the Vulcans in 2012, and was named 2012 New Zealand domestic player of the year. Matagi trialed with the Warriors during the 2013 pre-season, impressing coach Matthew Elliott enough to earn an NRL contract.
Matagi made his NRL debut for the Warriors on 11 May 2013 against the Canterbury-Bankstown Bulldogs at Westpac Stadium. On 11 June 2013, he re-signed with the Warriors until the end of 2014. Matagi played 11 games for the Warriors in 2013, scoring a try in their match against the Penrith Panthers in round 23. On 22 September 2013, Matagi was named at prop in the 2013 New South Wales Cup Team of the Year.
Matagi was a member of the Warriors' squad that played in the 2014 and 2015 Auckland Nines pre-season tournaments. On 2 March 2014, Matagi re-signed with the Warriors until the end of 2016. He played in all 24 of the Warriors' regular season games in 2014, but played in only 1 game during the first half of 2015 before being dropped to the Warriors' New South Wales Cup team. In May 2015, Matagi requested, and was granted, a release from his contract with the Warriors in order to sign with the Sydney Roosters effective immediately.
Stuff.co.nz and The Sydney Morning Herald described Matagi as having built a cult following during his time at the Warriors.
Upon moving to the Roosters, Matagi played for their feeder club, the Wyong Roos, in the New South Wales Cup. An injury to Jared Waerea-Hargreaves saw Matagi play 7 games for the Roosters in the back-end of 2015.
On 4 November 2015, Matagi signed a one-year contract with the Penrith Panthers, with an option for a second year. He was a late addition to the Panthers' 2016 Auckland Nines squad following an injury to Reagan Campbell-Gillard. Matagi was a regular for the Panthers' in 2016, playing a total of 23 games.
In November 2016, Matagi signed a two-year contract with the Parramatta Eels.
Matagi made his international debut for Samoa at the 2013 World Cup, playing in all four of their matches, scoring tries against New Zealand and Papua New Guinea. In May 2014, Matagi played for Samoa in the 2014 Pacific Rugby League test against Fiji to qualify for the 2014 Four Nations.
In October 2014, Matagi was named in both the New Zealand and Samoan Four Nations squads. He opted to represent New Zealand, playing in one of their four matches, against Samoa.
Matagi was named in New Zealand's train-on squad for the 2016 Four Nations, but was not included in the final team.
Matagi is a Christian. He is a father of four children with his partner Fai.