He lost in the quarterfinals of another tournament in Iran, where he made his Challenger singles final, partnered by Ludwig Pellerin, falling to Juan Ignacio Cerda and Jasper Smit in straight sets.

2005 saw Montcourt make his first Futures doubles final; losing it in March in 3 sets to Lesley Joseph and Evgeny Korolev, alongside Jean Baptiste Robin.

In 2006, Montcourt won through to back-to-back April semifinal Futures and performances and then went on to play his first major match by receiving a Wildcard into the 2006 French Open.

In doubles, he won three 3-set matches to win his first title with Gregory Carraz, beating Tomasz Bednarek and Frank Moser in the final of a Challenger event in Rennes.

In addition, Mathieu participated in a doubles Grand Slam event with Gregory Carraz, losing to Łukasz Kubot and Oliver Marach.

3 of the Centre National d'Entraînement at Stade Roland Garros was renamed le Court Mathieu Montcourt, in honor of the player who trained there regularly.