Brommapojkarna In 2010, he made his debut for Brommapojkarna in
Allsvenskan – the Swedish top tier – at age 18. He played 12 games in the league during his debut season, as Brommapojkarna got relegated to
Superettan. He established himself as a regular starter in the central midfield position for Brommapojkarna in 2012 and 2013, only missing a few competitive games. Martinsson Ngouali soon got known for his technical gift and strong passing game. Before the start of the 2014 season, he attracted interest from then reigning Swedish champions
Malmö FF. Martinsson Ngouali opted to stay at Brommapojkarna and signed a new three-year deal with the club. Brommapojkarna eventually declined and got relegated to Superettan before the start the 2015 season, where they would finish in last place. In 2016, Martinsson Ngouali scored 7 goals – a new career best – as Brommapojkarna won
Division 1, the Swedish third tier. Martinsson Ngouali signed a three-year deal with the Allsvenskan club. He rejoined his former manager
Stefan Billborn and former teammate
Pablo Piñones Arce at Hammarby, now both working as assistant managers at said club. He made his competitive debut for the club on 3 April, on the first match day of
Allsvenskan 2017. Martinsson Ngouali got sent off late in the second half as Hammarby lost 1–2 against IFK Norrköping. He scored his first goal for Hammarby on 7 May in an away fixture against
Östersund, netting in a 2–1 loss. Midway through his first season at Hammarby, Junior received much praise from the club's director of football
Jesper Jansson, who complimented him as a "real class player" with a strong defensive game and great ability to cover big surfaces on the pitch. On 21 August, in a 3–0 away win against
Örebro SK, Martinsson Ngouali attracted a serious thigh injury that kept him out of play for almost two months. He returned to the pitch on 16 October, in a 0–2 away loss against
Kalmar FF.
2018 On 12 February 2018, Junior extended his contract for another half year, with the new deal running until June 2020. He played 26 league games for Hammarby, scoring once, as the club finished 4th in the table. He suffered a serious cruciate ligament injury in October, in a 2–1 loss against Malmö FF, with an expected return in the summer of 2019. At the end of 2018, Martinsson Ngouali was voted Hammarby Player of the Year by the fans of the club and also featured in the official Allsvenskan Team of the Year.
2019 Martinsson Ngouali spent the first half of the 2019 season nursing his serious knee injury. He made his comeback on 15 September in a 6–2 home win against
IFK Göteborg. He eventually played 8 games, scoring once, as Hammarby finished 3rd in the table.
2020 He had difficulties breaking into the side as a regular in 2020, as the club disappointedly finished 8th in the table. On 9 December, it was announced that Martinsson Ngouali would leave the club at the end of the year, as his contract expired.
HNK Gorica On 15 February 2021, Martinsson Ngouali signed a one-and-a-half-year contract with
Prva HNL club
HNK Gorica, linking up with
Jiloan Hamad, his former teammate from Hammarby. ==International career==