Youth career Kirev began his career at
FC Germanea. His first coach was Georgi Stanchev. At the age of fourteen, he began training with the junior team of
Slavia Sofia.
Slavia Sofia In August 2007 Kirev signed his first professional contract with
Slavia. He made his competitive debut for the club on 25 April 2008 against
Spartak Varna in the twenty five round of the
Bulgarian top division. In next season (2008–09) Kirev was the first choice for goalkeeper of the coach
Stevica Kuzmanovski. He earned 7 appearances playing in the
A PFG.
Juventus In December 2008, Kirev was invited by
Serie A side
Juventus to join trial period. On 30 January 2009, he signed a four-and-a-half-year contract. In the following days Kirev was loaned for six months in the
Swiss team
Grasshopper Club Zürich. On 1 July 2009, he returned to Juventus. He was included in the
Primavera squad that winning a second consecutive
Torneo di Viareggio. However, he was the third keeper in the final. In February 2010, Kirev signed for
FC Thun in
Switzerland, in an attempt to gain first team experience. On 1 July 2010, Kirev returned to Juventus and became the 5th keeper, behind injured Buffon, new signing
Marco Storari,
Alex Manninger and new signing
Marco Costantino. He was called up to the first team, for the first time on 29 October 2010, against
Milan. In September 2011, Kirev joined
Romanian
Liga II side
Politehnica Timișoara on a season long loan deal. He returned to Juventus on 1 July 2012. On 6 July 2013 – Mario played in a friendly for Slovak side MFK Kosice against Hungarian side Debrecen, with MFK Kosice winning 4–1. This could signal a move to Slovakia and the end of his Juventus career.
Slavia Sofia After six years spent abroad, Kirev returned to
Slavia Sofia in the spring of 2015. His contract was terminated by mutual consent on 11 August 2017. ==International career==