In 2009, Mahrez joined CFA side Quimper from AAS Sarcelles, making 22 appearances and scoring 2 goals in his first season with the club.
He joined Le Havre in 2010, initially playing for second team, Le Havre II, before going on to play 60 times and score 6 goals for the first team in the French Ligue 2 from 2011 until leaving in January 2014.
He moved to English Football League Championship club, Leicester City on 11 January 2014, signing a three-and-a-half-year contract. Mahrez made his debut on 25 January 2014, coming on in the 79th minute as a substitute replacing fellow winger Lloyd Dyer, in the 2–0 win against Middlesbrough. After making four substitute appearances for Leicester, including scoring his first goal for the club, an 82nd minute equaliser against local rivals Nottingham Forest, manager Nigel Pearson announced in February 2014 that he thought Mahrez was ready to start games.
He made his Premier League debut on 16 August 2014, and scored his first Premier League goal on 4 October 2014.
In November 2013 Mahrez expressed his desire to represent Algeria internationally. He was called up to the provisional Algerian squad for the 2014 FIFA World Cup. On 31 May 2014, Mahrez made his international debut for the Desert Foxes as a starter in a pre-World Cup friendly match against Armenia, and he was subsequently called up to the full-squad for the tournament on 2 June. On 15 October 2014, Mahrez scored his first international goal, as well as setting up Islam Slimani in Algeria's 3–0, Africa Cup of Nations qualifier against Malawi. In December 2014 he was announced as being part of Algeria's squad for the 2015 Africa Cup of Nations.