Born in Melbourne, Victoria Fonua attended school at Hallam Senior College Sports Academy. Fonua played his junior football for the South Eastern Titans, Melbourne in the Victoria Rugby League, before being signed by the Canberra Raiders. In 2006 and 2007, Fonua represented the Victoria Under 15s team, he also represented the Victoria Under 18s team. In 2008 and 2009, he represented the Australian Capital Territory Under 18s team. He played for the Raiders' SG Ball team as the Storm did not have an SG Ball team at that time, before returning to Melbourne after being signed by the Melbourne Storm.
Fonua played for the Storm's NYC team from 2010 to 2012, scoring 32 tries in 45 games. At the end of the 2011 season, Fonua was named at centre in the 2011 NYC Team of the Year.
In Round 23 of the 2012 NRL season, Fonua made his NRL debut for the Storm against the Gold Coast Titans on the wing in the Storms 24-16 win at AAMI Park. Fonua became the first Victorian born and bred player to represent the Storm in first-grade. In Fonua's third game for the Storm in the Finals 1 week match against the South Sydney Rabbitohs, Fonua scored his first NRL career try in the Storms 24-10 win at AAMI Park, after previously being denied tries on four occasions by the video referee, once in his debut, twice in his second game, and once prior to his try in his third game. Fonua finished off his debut year in the NRL with him playing in 4 matches and scoring a try in the 2012 NRL season. On 2 October 2012, Fonua was named at wing in the Junior Kangaroos team to face the Junior Kiwis.
Fonua played in Storm's 2013 World Club Challenge victory over Leeds Rhinos. Fonua was selected to play for his parents' country of origin Tonga on 20 April 2013 against Samoa in the Pacific Rugby League International at Penrith Stadium. He scored 2 tries in the Tongan 36-4 victory. On 10 June 2013, Fonua extended his contract with the Storm for a further two seasons, keeping at the club till the end of the 2015 season. In Round 21 against Canberra Raiders at Canberra Stadium, Fonua scored a hat-trick of tries in the Storms 68-4 win including a spectacular set up for Will Chambers after the ball seemed almost certain to go dead, with some commentators comparing it to the famous set up by Greg Inglis during the centenary test. On 8 October 2013, Fonua was selected in the Tonga 24 man 2013 Rugby League World Cup squad, playing in 2 of Tonga's 3 matches.
On 14 February 2014, Fonua was selected in the Storms' inaugural 2014 NRL Auckland Nines squad. Fonua finished the Storms' 2014 NRL season with him playing in 20 matches and scoring 10 tries.
On 2 May, Fonua played for Tonga against Samoa in the Polynesian Cup, playing on the wing in Tonga's 16-18 loss at Cbus Super Stadium. He finished his last season with the Melbourne Storm having played in 13 matches and scoring 4 tries in the 2015 NRL season. On 17 October, he represented Tonga again, this time in their Asia-Pacific Qualifier match against the Cook Islands for the 2017 Rugby League World Cup.
On 1 August, he signed a 3-year contract with Super League team Hull F.C. starting in 2016. Fonua had an outstanding first season with the club and quickly established himself as a fan favourite. He helped Hull F.C. reach the Challenge Cup Final where they would go on to beat Warrington Wolves by a narrow score of 12-10. Fonua scored the first of Hull's two tries. In his first season in Super League, he scored 15 tries which contributed to him being selected for the Super League Dream Team along with 5 other Hull teammates.
On 9 June 2017, it was confirmed that Fonua would return to the NRL, to join the Wests Tigers, starting in the 2018 season.
On 26 August 2017, Fonua would go on to win the Challenge Cup for a second year in a row, contributing massively to the 18-14 win over Wigan Warriors by scoring two of Hull's three tries.World Club Challenge: (1) 2013
Challenge Cup: (2) 2016, 2017
Super League Dream Team: (1) 2016