On 14 February 2014, Brown was selected in the Cronulla-Sutherland Sharks inaugural 2014 NRL Auckland Nines squad. Brown was selected to play for Samoa against Fiji in the 2014 Pacific Rugby League International test, but withdrew due to injury. On 5 June 2014, Brown re-signed with the Sharks on a 3-year contract, two days before his NRL debut. In Round 13 of the 2014 NRL season, Brown made his debut for the Sharks against the St. George Illawarra Dragons playing at five-eighth in the Sharks 30–0 loss at WIN Stadium. In Round 21 against the Parramatta Eels at Remondis Stadium, Brown scored his first NRL career try in the Sharks 32–12 loss. Brown finished his debut year in the NRL with him playing in 11 matches and scoring a try for the Sharks in the 2014 NRL season. On 9 September 2014, Brown was selected for the New Zealand and Samoa 2014 Four Nations train-on squads, but didn't make the final 24 man squads for both teams.
On 7 May, Brown made his international debut for Samoa against Tonga in the 2016 Polynesian Cup, where he played at five-eighth and kicked 3 goals in a man-of-the-match performance. Brown spent most of the season with foundation club and feeder side to Cronulla, The Newtown Jets, where he was part of the side that made it to the preliminary final before losing to eventual premiers Illawarra.
2017
On 17 May 2017, Fa'amanu Brown signed a two year deal with the Canterbury-Bankstown Bulldogs. After waiting almost a year for a chance to be recalled to the side, Brown finally made his NRL return in Cronulla's clash with Canterbury in round 12, with the regular half, James Maloney, unavailable due to State of Origin duties. On 21 July 2017, Brown scored his first try of the season for Cronulla against Souths in a 26–12 victory. On 28 July 2017, Brown started for Cronulla against the New Zealand Warriors and scored a try, but was later taken from the field after suffering a head collision.
2018
Brown made his debut for Canterbury-Bankstown in Round 3 against the Penrith Panthers. Brown made a total of 9 appearances in 2018 as Canterbury spent the entire season down towards the foot of the table. At one stage, the club were running second last on the table before upset wins over the Brisbane Broncos, New Zealand Warriors and St George lifted the club up the table to a 12th place finish. Brown played in Canterbury's Intrust Super Premiership NSW grand final victory over his former club Newtown at Leichhardt Oval. The following week, Brown played in Canterbury's NRL State Championship side which defeated Redcliffe 42-18 at ANZ Stadium.
2019
Brown only made 3 appearances for Canterbury in the 2019 NRL season. For most of 2019, Brown played for Canterbury's reserve grade team in the Canterbury Cup NSW competition. Brown's last game for the club was the qualifying final against St George which Canterbury lost 24-22 at Campbelltown Stadium. On September 16, it was revealed that Brown would be released from the club after not being offered a new contract for the 2020 season.
