In 2013 and 2014, Mata'utia played for the Knights' NYC team. On 16 October 2013, he extended his contract with the Knights from the end of 2014 to the end of 2015. On 12 June 2013, he won the Brian Carlson Club-Andrew Johns Medal as the Knights' best under-17 player. In September and November 2013, he captained the Australian Schoolboys.
2014
On 3 May, Mata'utia played for the New South Wales Under-20s team against the Queensland Under-20s team, playing at centre and scoring a try in New South Wales' 30-8 win at Penrith Stadium. In round 20 of the 2014 NRL season, he made his NRL debut for the Knights against the Sydney Roosters at fullback, alongside his brother Chanel Mata'utia who also debuted for the Knights in the same game. This was the first time since Round 1 of the 1942 NSWRFL season, that two brothers had debuted in the same game together, Bill and Doug McRitchie debuting for the St. George Dragons on that day. In round 23 against the New Zealand Warriors at Hunter Stadium, he scored his first NRL career try in a 3 try effort for the Knights during their 28-22 win. He finished off his debut year in the NRL having played in 7 matches and scoring 7 tries for the Knights in the 2014 season. On 2 September, he was named at centre in the 2014 NYC Team of the Year. On 8 September 2014, he was named in the Samoa train-on squad for the 2014 Four Nations, later being also named in the Junior Kangaroos train-on squad. On 12 October 2014, he played for the Prime Minister's XIII against Papua New Guinea. On 14 October 2014, he was named in the Australian 2014 Four Nations squad. He became Australia's youngest ever Test player when he played against England, aged 18 years and 129 days, eclipsing Israel Folau's seven-year-old record by 65 days. He played in 3 matches for Australia including playing on the wing and scoring a try in the Kangaroos' 22-18 loss to New Zealand at Westpac Stadium. Late in September 2014, he signed a letter of intent to join the Canterbury-Bankstown Bulldogs on a four-year contract starting in 2016, along with his brothers Chanel and Pat. However, the NRL's rules didn't allow the Bulldogs to register the contracts until 30 June 2015, leaving the option of staying at the Knights open for the Mata'utias.
2015
On 31 January and 1 February, Mata'utia played for the Knights in the 2015 NRL Auckland Nines. On 19 March, he re-signed with Newcastle on a three-year contract, along with his brothers Chanel and Pat. He finished off the 2015 season having played in 22 matches and scoring 3 tries as the club finished last.
2016
In February, Mata'utia played for the Knights in the 2016 NRL Auckland Nines. In round 17 against the Canberra Raiders, he switched from the centres to second-row in the Knights' 21-25 golden-point extra time loss at Canberra Stadium. This would later prove to be a permanent move for the rest of the season. He finished the 2016 season having played in 16 matches and scoring 4 tries.
