An Algerian international, Slimani made his international debut in 2012 and played at the 2013 Africa Cup of Nations and the 2015 Africa Cup of Nations. He was also part of Algeria's squad at the 2014 FIFA World Cup, where he finished as the team's joint top scorer with two goals. As of January 2017, he has won 49 international caps and scored 25 goals, making him Algeria's fourth highest all-time goalscorer.
In May 2009, Slimani joined CR Belouizdad from JSM Chéraga for a transfer fee of 800,000 Algerian dinars, signing a two-year contract. In August, he made his official debut for the club as a starter against MC Oran in the first week of the 2009–10 Algerian Championnat National. He finished his first season with Belouizdad with 8 goals in 30 games.
On 17 May 2011, Slimani scored four goals in a league game against JS Kabylie, leading CR Belouizdad to an impressive 7–1 win. In July 2011, with his contract expired and several French clubs such as Nice and Le Havre showing interest in signing him, Slimani decided to sign a two-year extension with CR Belouizdad.
On 6 August 2013, Slimani joined Sporting Clube de Portugal for an undisclosed fee. Slimani started the 2013–14 season as a substitute, gaining a reputation as an impact player due to him scoring vital goals when coming off the bench. Due to the lack of form of Fredy Montero, however, he became a starter and scored four goals in four matches in early March 2014 against Rio Ave, Braga, Vitória de Setúbal and in 1–0 over rivals Porto. In December 2013 he was considered the best Algerian footballer of the year as he won the Algerian Golden Ball.
The second season Slimani was successful as he became a key player, for the first goal in the season was in the Derby de Lisboa against Benfica. outside the home and finished with a draw 1–1, while on the European level Slimani scored his first goal against Schalke 04 at Estádio José Alvalade in the last minute to finish 4–2. for Sporting, at the end of the season he finished third in the league but in the Taça de Portugal he reached the final against Braga on 31 May 2015, slimani scored a goal helping Sporting turn around a two-goal deficit to eventually secure the cup following a penalty shootout 3–1. where he achieved his first title in his history at the end of the season Slimani played 33 games and scored 15 goals, including 12 in the league.
In the 2015-16 season, Slimani said he would be his last season and aspires to achieve the title of the Primeira Liga absent for 14 years the new and former Benfica coach Jorge Jesus, who said he would rely heavily on Slimani to win the titles and the start was in the Supertaça against Benfica and ended for Sporting with a single goal, achieving his second title. at the start of the tournament, Slimani scored his first goal against Académica outside the home and on 4 October 2015, in the seventh round against Vitória de Guimarães scored his first hat-trick in the Portuguese league. at the top of the 15th round against Porto, Slimani managed to score two goals against goalkeeper Iker Casillas. to lead the team back to the top of the league. after this game Slimani scored in four consecutive games 6 goals against Vitória de Setúbal, Braga, Tondela and Paços de Ferreira. after two matches he returned to score against Nacional twice outside the home. and before the derby match against Benfica the club had lodged a complaint against Slimani for assault the striker Konstantinos Mitroglou in the Taça de Portugal on 21 November 2015, after a long wait and a day before the match, the Portuguese League announced that Slimani could participate in the decisive match for the title. however, the team lost 1–0 to lose the lead. One week later, He scored his 20th goal this season in the match against Estoril. after this meeting scored four goals against Belenenses, Marítimo and Moreirense and in the rivalry against Porto, Slimani contributed to his team's victory in Estádio do Dragão after scoring two goals. In the final round he scored the 27 goals against Braga The match ended 4–0 but was not enough to win Sporting league title and just two points behind Benfica and the same thing for the top goalscorers that returned to the Brazilian Jonas With 32 goals and Slimani with 27 goals only in this season Slimani scored 9 goals against the Big Three Porto, Benfica and Braga in 9 matches. in the new season, Sporting wanted to retain Slimani but the player insisted on going, which was finally accepted by the administration. On 28 August 2016, he played his last match against Porto at Estádio José Alvalade and scored his last goal to finish 2–1 for Sporting. at the end of the match, Slimani leaves the stadium with tears and he thanked all the fans.
On 31 August 2016, the last day of the 2016–17 summer transfer window in England, Slimani joined Premier League club Leicester City on a five-year contract. The transfer fee paid to Sporting was reported as £28 million, a club record for Leicester. Leicester had beaten West Bromwich Albion to sign Slimani.
On 14 September, he made his debut for Leicester in the UEFA Champions League playing 62 minutes in a 3–0 victory over Club Brugge. Three days later, he made his Premier League debut in a 3–0 victory over Burnley, scoring his side's first two goals. On 27 September, Slimani scored the only goal in Leicester's 1–0 win against Porto in the Champions League group stage after a pass from Algerian national teammate Riyad Mahrez. It was his sixth goal with Iker Casillas in goal in 2016, five of which he netted while with his former club Sporting.
After this goal, Slimani failed to score until matchday 11 of the Premier League against West Bromwich Albion from a pass from Mahrez, although Leicester lost 2–1. Later, on matchday 13, after entering as a substitute, Slimani scored the equalizer for his team from the penalty spot in the last minute against Middlesbrough. On matchday 15, Slimani helped the Foxes to win against Manchester City 4–2 after he presented two assists for goalscorer Jamie Vardy and Andy King. On 31 December, after five games without a goal, Slimani scored the winning goal against West Ham United at the King Power Stadium to lead his team to win the fifth in the Premier League this season, the last day of 2016.
Slimani scored four of his five league goals from headers, but has only managed a 38% shot accuracy by January 2017. after his participation in the African Cup, Slimani returned to the club injured, which affected the rest of the journey this season where he did not participate very much and scored only two goals against Sunderland. and the second against Everton. Slimani scored 8 goals from 29 games, a poor result compared to his value, In the new season 2017-18, Leicester City wanted to sell Slimani but his big physical value did not help. The manager Craig Shakespeare said he did not want him but considered him an excellent striker. however, in the League Cup match against Sheffield United, he scored two goals.
In October 2009, Slimani was called up by Abdelhak Benchikha for the first time to the Algeria A' National Team for a week-long training camp in Algiers. In March 2010, Slimani was called up again, this time for a 2011 African Nations Championship qualifier against Libya as a replacement for his injured CR Belouizdad teammate Youcef Saïbi. However, Slimani did not play in the game as Algeria won 1–0.
In May 2012, Slimani was called up for the first time to the Algeria national team for the 2014 FIFA World Cup qualifiers against Mali and Rwanda, and the return leg of the 2013 Africa Cup of Nations qualifier against Gambia. On 26 May, he made his debut, coming as a substitute at half-time in a friendly against Niger. A week later, on 2 June, Slimani scored his first international goal for Algeria, netting the third goal in the 4–0 win over Rwanda in the 2014 FIFA World Cup qualifiers. He followed that up with another goal against Mali in the next game, and then scored another two goals in the following against Gambia.
Slimani was selected for Algeria's squad taking part in the 2013 Africa Cup of Nations, and played in all three group games in January. Algeria, however, only won one point and were eliminated at the first stage.
Algeria qualified for the 2014 World Cup, with manager Vahid Halilhodžić selecting Slimani for his 23-man squad. On 22 June 2014, Slimani scored the opening goal for les Fennecs in a 4–2 group match against South Korea. He also made an assist for the team's third goal by passing the ball to Abdelmoumene Djabou. On 26 June, Slimani scored with a header in Algeria's 1–1 draw with Russia, helping the nation qualify for the knockout rounds of the World Cup for the first time ever after failing to do so previously in 1982, 1986 and 2010. In the round of 16 match against Germany, Slimani had a headed goal disallowed in the first half for offside, while later on, his powerful shot at goal was saved by German goalkeeper Manuel Neuer. Algeria eventually lost 2–1 after extra time, but were praised for an excellent campaign throughout the tournament.
In Algeria's opening match of the 2015 Africa Cup of Nations, Slimani scored the final goal in a 3–1 victory over South Africa.As of match played 20 September 2017
As of match played 11 June 2017
Scores and results list Algeria's goal tally first.
Taça de Portugal: 2015
Supertaça Cândido de Oliveira: 2015
Algerian Ballon d'or: 2013
CAF Team of the Year: 2016
SJPF Player of the Month: December 2015