Mathias' compulsory military service began on 17 May 1950. A few months later, he enlisted in the first contingent. At the end of his first mission, he joined the second contingent in their training camp in Namur. There, the experience he had gained on the Korean battlefield proved to be of great value for the inexperienced soldiers of the second contingent. In March 1952, he left once again to resume combat on the Korean peninsula. During his second term, in July 1952, a bullet hit his right arm, heavily injuring him. He would have died if it had not been for the Swedish Red Cross to evacuate and for a Norwegian MASH to treat him.

On his return, he stayed in the Army until his official discharge on 13 July 1960.