Mohsni is of Tunisian descent and grew up in the Les Ulis suburb of Paris, France.
In his homeland, he played for CO Les Ulis (twice), Mende, US Saint-Georges and Sainte-Genevieve Sports at the beginning of his career, between the years of 2005 and 2010. He combined playing part-time with coaching children in the community.
In the summer of 2010, Mohsni moved to England on the advice of his agent to sign with Southend United of League Two. He was one of seventeen new signings, as new manager Paul Sturrock rebuilt the side following a turbulent period in the club's history which left the club with only a handful of players and in the bottom rung of England's professional football ladder. This followed a successful trial period. This move came despite difficulties in securing his transfer from the French club, after delays in getting the necessary paperwork from the French Football Federation. In September 2010, he signed a new two-year deal with the club.
Mohsni made his Southend debut and his first game in professional football against Wolverhampton Wanderers in the League Cup on 24 August 2010. Mohsni had a goal disallowed for offside and hit the crossbar in the game Southend eventually lost 2–1 after extra time. On 27 August, Mohsni made his league debut against Bradford City. Mohsni scored his first goal for the club on 13 November 2010 in a 1–1 draw against Accrington Stanley. On 21 January 2011 Mohsni scored a 96th-minute equaliser against Bury to salvage a 1–1 draw. In the following game at home to Macclesfield Town, Mohsni was picked to start up front following his leveller against Bury and an injury to Barry Corr. He scored twice in a 4–1 win, with involvement in one of the other goals. On 31 January 2011, Southend rejected a bid of £250,000 from Blackpool for Mohsni. On 5 February 2011 against Shrewsbury Town Mohsni was stretchered off amidst fears he had suffered a broken ankle, scans later revealed no break or fracture but he had suffered ligament damage and would be out of action for at least two months. Mohsni made his return from injury on 2 April 2011 playing the full 90 minutes in a 0–0 draw against Aldershot Town. His return to first team action also saw him suffer a broken nose after being elbowed by a player.
Starting the 2011–12 season as a striker due to injuries, Mohsni started playing as a centre forward. He scored the only goal in a 1–0 opening day win at home to Hereford United. On 3 September 2011, again playing as a makeshift centre forward, Mohsni scored after a 35-yard strike against Northampton Town. He was later utilised as a winger for the majority of the 2011–12 season, and praised by manager Paul Sturrock for his versatility. However, Mohsni's on-field discipline was poor, resulting in a series of red and yellow cards. In December 2011 he was sent off for third time since joining Southend, after already being given one "last chance" due to his many suspensions. Manager Paul Sturrock expressed his exasperation by describing Moshni as a "walking foul". On 5 January 2012, Southend manager Sturrock confirmed that he would not be sold in the January transfer window, and would remain at Southend until the end of the season. On 20 April 2012, Mohsni scored a hat-trick in Southend's 3–0 win over Barnet, a result which secured their place in the promotion play-offs. Southend, however, were knocked out of the play-offs by Crewe Alexandra.
Mohsni failed to return for the start of pre-season training ahead of the 2012–13 season. Southend manager Sturrock announced that Mohsni was 'reluctant' to return to the club at all believing he could play his football at a higher level than Southend's League Two and that he was scheduled to go on trial with 'a Premier League side'. On 20 July he appeared for a West Ham United development side in a 4–1 defeat to Grays Athletic, scoring both a goal and an own-goal. In August 2012, West Ham's bid of £150,000 for Mohsni was rejected by Southend.
Mohsni was not given a squad number by Southend before the start of the 2012–13 season. On 28 August 2012, it was announced that Mohsni was to go on trial with SPL club Celtic.
He then spent a week on trial with Championship club Ipswich Town in September, however, it was later reported that Ipswich would not be signing Mohsni since he refused an extended trial at Portman Road. Mohsni then spent a fortnight on trial with Birmingham City before eventually securing a three-month loan deal with Ipswich Town on 5 October 2012. He played only five games for Ipswich, all substitute appearances, and found first team football opportunities limited due to restrictions on the number of on-loan players a team can field.
Having returned from Portman Road on 31 December 2012, Mohsni refused to train with the Shrimpers although he remained registered with the club. In January 2013, Mohsni spent time on trial with Stevenage, training with the squad, although no move materialised.
Eventually, on 16 February 2013, when Southend were desperately short of players through injury and suspension, particularly centre-backs, Mohsni agreed to play for the Blues, wearing the number 28 shirt. He made his return that day in a 2–1 defeat to Northampton, which only lasted 27 minutes after a high footed challenge saw him sent off. He was reported to have watched the remainder of the game from the Roots Hall stands, still wearing his kit. Bilel's final appearance for Southend came at Wembley stadium in the Football League Trophy Final, a competition he had played no previous part in. On 30 April 2013 Mohsni was released by Southend, just 10 months after the club turned West Ham's £150,000 offer for him.
On 4 July 2013, Mohsni began a week's trial at Rangers. The trial was extended by three weeks due to impressive performances in the 2013 pre-season campaign. On 26 July he signed a two-year deal with the club which was due to commence on 1 September when the club's transfer embargo had been lifted. He made his debut for Rangers at Ibrox on 14 September 2013 in a 5–1 win over Arbroath in the league. Mohsni became a regular in the Rangers side, playing in 28 league matches and scoring 10 goals.
Mohsni's ill-discipline flared up again on 2 August when he was sent-off in a pre-season friendly against Derby County. The defender was red carded for headbutting striker Chris Martin, and as a result received a two-match ban from the SFA for the first two competitive domestic fixtures of the season. Mohsni returned to the Rangers side after his suspension but found himself in further trouble as a result of an altercation with Hibernian defender Liam Fontaine during Hibs' 3-1 win at Ibrox on 29 September 2014. Mohsni was charged with violent conduct days later by the SFA Compliance Officer, and was suspended for three matches. On 31 May 2015 at Fir Park, in what was to be Mohsni's final game for Rangers, his time with the club ended in controversy following an altercation with Motherwell's Lee Erwin, with Erwin pushing Mohsni in the back. He retaliated by kicking and punching Erwin. He also fought with Motherwell substitute, Fraser Kerr. Mohsni, Erwin and Kerr were all shown red cards by referee Craig Thomson after the game had finished. On 1 June, Police Scotland announced an investigation into the incident at Fir Park. On 2 June, the Scottish Football Association revealed that Lee Erwin had only received a yellow card from referee Thomson for his part in the incident. Mohsni later received a seven match ban for the offences.
Mohsni returned to French football in August 2015 when he signed a two-year contract with newly promoted Ligue 1 club Angers SCO.
After six months in Angers without playing with the first team, Mohsni left Angers in January 2016 and signed a six-month contract with Ligue 2 club Paris FC. He joined ES Sahel in September 2016.
Mohsni was born and raised in France, but is also of Tunisian descent where his father was born. He was named in Tunisia's international squad for their friendly match against South Korea on 28 May 2014, where he made his international debut in a 1–0 win.
Tunisia coach Georges Leekens selected Mohsni for the 23 man squad taking part in the 2015 Africa Cup of Nations. However, Mohsni was an unused substitute in their opening 1–1 draw against Cape Verde and he had to deny reports of quitting the tournament after storming out of the dressing room at full-time. Tunisia reached the quarter-finals, where they lost 2–1 after extra-time to host nation Equatorial Guinea. Mohsni however did not feature in any of Tunisia four matches at the tournament.
After an absence from the team of over 18 months, Mohnsi played against Togo on 29 March 2016 in a qualifier tie for the 2017 Africa Cup of Nations.
Mohsni is a devout Muslim. During his time at Rangers, a room was set aside at both Murray Park (Rangers' training complex) and Ibrox Stadium to enable him to pray at the required times throughout the day. He was also permitted to arrive at a later time for training on Fridays to allow him to attend his mosque.
Southend UnitedFootball League Trophy runner-up: 2013
RangersScottish League One: 2013–14