Mitrev came through the youth ranks at Levski Sofia. For 2006–07 season he was promoted to the first squad as third choice behind Georgi Petkov and Nikolay Mihaylov. He made his debut in a 3–0 home loss against Werder Bremen in the group stage of the Champions League on 31 October 2006, replacing Nikolay Mihaylov at half-time. On 4 November, he started for first time, keeping a clean sheet in a 3–0 home league win over Beroe Stara Zagora. On 5 December, Mitrev played in Levski's 2–0 defeat to Chelsea at Stamford Bridge, making several excellent saves, including one reaction save from a Michael Essien header. However, he received criticism from the Chelsea faithful for a bad challenge on Didier Drogba, clearly fouling him. Chelsea were not offered a penalty, however Drogba accepted the apologies of the goalkeeper. Mitrev collected his first A Group title winner's medal at the end of the 2006–07 season, playing 12 matches in all competitions.