Diallo began his football career playing for his hometown club Stade Reims, joining the club as a youth player. In 2004, he was selected to attend the Clairefontaine academy and spent three years at the facility training at the academy during the weekdays and playing for Reims on the weekend. Prior to leaving Clairefontaine, Diallo agreed to join the nationally recognized youth academy of Stade Rennais on an aspirant contract. Rennes had been scouting the player since 2004.
Rennes
Diallo joined Rennes and was among a host of internationally recognized youth talents the club had comprised. These talents included youth internationals Elliot Sorin, Wesley Yamnaine, Jérémy Helan, who played for Manchester City in England, Abdoulaye Doucouré, and Axel Ngando. Diallo initially joined the club's under-18 team who were playing in their own league, the under-18 Championnat National. Rennes finished in 4th-place position, missing out on the playoffs. The 2008–09 season saw Diallo earn a place in the club's Coupe Gambardella team with the club attempting to defend their title from the previous season. In the tournament, the goalkeeper appeared in three matches and conceded four goals. Rennes suffered elimination in the Round of 16 losing to Le Havre. Diallo was promoted to the club's Championnat de France amateur team for the 2009–10 season. He made his debut on 15 August 2009, earning a clean sheet in the team's 2–0 victory over US Sénart-Moissy. Diallo appeared in four more matches, before earning a call-up to the senior team on 20 November by manager Frédéric Antonetti for their league match against Le Mans in place of injured backup goalkeeper Cheick N'Diaye. Antonetti preferred the 17-year-old ahead of Patrice Luzi and Florent Petit, the third and fourth goalkeeper, respectively. Diallo appeared on the bench in the Le Mans match, with Rennes earning a 2–1 victory. The following week, Diallo was named the starting goalkeeper for the club's league match against Lyon, almost an hour before the match was set to be played, due to an injury picked up by starting goalkeeper Nicolas Douchez the previous day. Diallo started the match and played the entire 90 minutes in a 1–1 draw, making spectacular saves, including an important save in the 88th minute off a shot by Bafétimbi Gomis. On 18 April 2010, Diallo signed his first professional contract, agreeing to a three-year deal.
Gençlerbirliği
On 28 June 2019, Diallo signed with Turkish club Gençlerbirliği.
