Monaco Nkoulou began his career at
Kadji Sports Academy in
Douala, the same academy that produced the likes of
Samuel Eto'o,
Carlos Kameni, and
Eric Djemba-Djemba. In late 2007, it was announced that he was acquired by the
principality-based side
AS Monaco. He agreed to a three-year professional contract with the club. He spent the latter portion of the 2007–08 season playing with Monaco's
CFA squad, before joining the squad for preseason in July 2008, where he was handed the number 3 kit. Nkoulou made two appearances during the pre-season starting their opening friendly match against fellow Ligue 1 side
Toulouse and also starting against Croatian side
NK Zagreb. He made his professional debut on 13 September 2008 in a 2–0 victory over
Lorient starting the match before being substituted in the 65th minute. Though he made 24 appearances in the 2008–09 season, The 2009–10 season saw Nkoulou to play as defensive midfielder for most of the season under the new management of
Guy Lacombe and new signing,
Sébastien Puygrenier. As the 2009–10 season progressed, Nkoulou signed a new contract with the club, keeping him until 2012 and was then reverted to central defence. NKoulou also played in the
Coupe de France Final, coming on as a substitute for
Eduardo Costa in the 111th minute, in a 1–0 loss against
PSG and went on to make 23 appearances in the 2009–10 season despite being committed to the African Cup of Nation. In the 2010–11 season, Nkoulou found himself in the transfer speculation and ended up staying at the club despite fallen out with Lacombe. Nkoulou continued remain in the first team, playing in the defensive midfielder position for most of the season and central defence. Despite suffering from injuries and suspension, Nkoulou finished the 2010–11 season, making 30 appearances. However, following their relegation to
Ligue 2, Nkoulou was among key players to be expected to leave the club. His performances with Monaco have led to him being scouted by several prominent European clubs. English club
Arsenal, Spanish sides
Sevilla and
Atlético Madrid and French champions
Olympique Lyonnais have all declared their interests in the Cameroonian defender. During his time at AS Monaco, Nkoulou made 78 appearances and has started every match since playing the full 90 minutes in them all. He has yet to score a goal.
Marseille . On 29 June 2011, Nkoulou passed his medical and signed for four years with
Marseille for €3.5 million. Marseille and Lyon found themselves competing over their interest signing Nkoulou before he chose Marseille. After missing two matches, due to suspension he received while at Monaco, Nkoulou made his Marseille debut on 14 August 2011, in a 2–2 draw against
Auxerre. In the next match against
Saint Étienne on 21 August 2011, Nkoulou was involved in an incident with
Florent Sinama Pongolle. But following the reviewing by the Disciplinary Committee of the LFP, he was received a three match ban. While serving his suspension in the league, he made his Champions League debut on Matchday 1 of the Group–stage, helping a club kept a clean sheet, in a 1–0 win against
Olympiacos on 13 September 2011 and went on to play ten Champions League Matches. Eight days later on 21 September 2011, Nkolou made his return to the first team, in a 2–0 win over
Evian. His performance throughout October saw him earned the club's player of the month of October. Nkolou continued to remain in the first team for the 2011–12 season despite another suspension (which he got a two match ban against
Dijon) and a knee injury. Despite this, Nkoulou helped the club win against Lyon 1–0
in the final of Coupe de la Ligue, winning the cup for their first piece of silverware since 1981, as well as, being named Man of the Match. After finishing the 2011–12 appearance, making 46 appearances in all competition, Nkoulou was named
Trophées UNFP du football's Team of the Year of
2012 and was nominated for the
Prix Marc-Vivien Foé Award, but finished as runner–up to
Younès Belhanda. Nevertheless, Nkoulou was awarded 2011–12 Olympique de Marseille Player of the Season. In the 2012–13 season saw Nkoulou found himself in the transfer speculation when the club began to suffer financial trouble, but stayed at the club and insisting on honouring his contract. Nkoulou was ever–present in the 2012–13 season, having played all the match in the league and scored his first goal for the club, in a 2–1 win against
Troyes on 3 March 2013. During the season, Nkoulou suffered a knee ligament injury and was sidelined for weeks, but managed to return to the league. At the end of the 2012–13 season, Nkoulou was named
Trophées UNFP du football's Team of the Year of
2013 for the second time running. in 2012. Prior to the 2013–14 season, Nkoulou stated he was prepared to leave Marseille in the summer, prompting attracted interests from his former club Monaco, who were just promoted back to Ligue 1, and Arsenal. After the club set a higher price tag on him, Nkoulou remained at the club and started a bad start when he conceded a goal, in a 3–1 win over
Guingamp in the opening game of the season. Despite this, Nkoulou redeemed himself soon after to continuously remain in the first team under the new management of
Élie Baup. On 4 January 2014, Nkoulou extended his contract with Marseille, keeping him until 2016. As the 2013–14 season progressed, Nkoulou suffered a knee injury, which kept him sidelined for one match. After making his return, Nkoulou went on to score two goals later in the season against Saint Étienne and
Sochaux. Nkoulou made 46 appearances and scoring twice in all competitions in the 2013–14 season. Ahead of the 2014–15 season, the club were keen on selling Nkoulou, but following a friendly match against
Bari, it appeared hinted that he wanted to stay at the club. His good performance against Guingamp on 23 August 2014 earned him Team of the Week on Matchday 3. Then on 19 October 2014, Nkoulou scored his first goal of the season, in a 2–0 win over
Toulouse. Two weeks later, on 2 November 2014, Nkoulou scored again, in a 2–1 win over
Lens. However, as the 2014–15 season, his appearance was soon restricted to 26 appearances in competition, due to injuries and international commitment with Cameroon. After two months without playing, Nkoulou made his first team return on 12 April 2015, in a 1–0 loss against
Bordeaux and went on to help the finished the season in the 4th place at two points of a qualification in the Champions League. Ahead of the 2015–16 season, Nkoulou's contract set to expire and haven't renewed his contract, though his future remained undecided. In the 2015–16 season, Nkoulou made his first appearance of the season, having missed through opening game of the season, in a 1–0 loss against
Reims on 16 August 2015. In a match against
Lille on 29 January 2016, Nkoulou captained Marseille for the first time following
Steve Mandanda's absence, which they drew 1–1. In a match against Saint-Étienne on 21 February 2016, Nkoulou received a straight red card in a 1–1 draw. In addition to his disciplinary issues, Nkoulou was suspended three times in every
Coupe de France matches, Nkoulou started in the
Coupe de France Final, which they lost 4–2 against PSG. At the end of the 2015–16 season, the club faced a difficult season with three different managers during the season and finished the league in thirteenth place and finished the season, making 41 appearances in all competition. With his contract expiring at the end of the 2015–16 season, Nkoulou attracted interests around clubs, such as
Málaga,
Schalke 04,
Juventus and
Lazio. However, on 27 June 2016, Nkoulou began talks with a move to stay in France by joining Lyon.
Lyon After spending five years at Marseille, with his contract at the club expired, Nkoulou moved to
rivals Lyon on a four–year contract on 29 June 2016; coincidentally was the day he joined Marseille. Upon joining the club, Nkoulou stated he joined Lyon to achieve greater things and was given number 3 shirt, which he quoted: ‘’“The No. 3? It is a wink to those close to me, because I am the youngest of three children.”’’ Nkoulou played his first official match for Lyon at the
Trophée des Champions, making his first start, in a 4–1 loss against Paris Saint–Germain. A week later, he made his league debut for the club, in the opening game of the season, in a 3–0 win over
Nancy. In his first match against Marseille for the first time, which saw them draw 0–0 on 18 September 2016, Nkoulou received boos from Marseille supporters during the match every time he touched the ball. Around the same time he was competing with
Mouctar Diakhaby, Nkoulou's performance soon came under criticism by observers, prompting Manager
Bruno Génésio defended him.
Torino On 5 August 2017, Italian club
Torino announced they had signed Nkoulou on a year-long
loan with an option to purchase. He made his debut on 11 August 2017, in the
Coppa Italia against
Trapani, won by Torino 7–1. His debut in
Serie A took place instead on 20 August, against
Bologna (1–1). He scored his first goal for the club on 2 December, during a 1–1 draw against
Atalanta. At the end of his first season with the club, he was voted by the fans as the best player of Torino in the 2017–18 season. His contract was redeemed by Torino for €3.5 million. On 2 September 2018, he scored his first goal of the 2018–19 season, and the match-winner with a header against
SPAL (1–0). In August 2019, Nkoulou asked to not be selected for Torino's Serie A fixture against
Sassuolo, and their
UEFA Europa League play-off against
Wolverhampton Wanderers, with a desire to transfer to
AS Roma. In September, with no transfer taking place, he wrote a letter saying he had decided to "leave the difficult moment of the summer behind me", and apologised to Torino fans, staff, and his teammates. At the end of the 2020–21 season, Nkoulou parted ways with
Torino upon the expiry of his contract.
Watford After spending time as a
free agent, on 7 October 2021,
Premier League club
Watford announced the signing of Nkoulou on a deal until the end of the season. Following relegation at the end of the
2021–22 season, Nkoulou was released by the club.
Aris On 23 August 2022, Nkoulou joined
Super League Greece club
Aris on a free transfer on a two-year contract.
Gaziantep On 11 September 2023, Turkish
Süper Lig club
Gaziantep announced the signing of Nkoulou on a one-year contract. ==International career==