[4] The dates of his birth and death are unknown, and his antecedents are unclear, but neither his name (which is found in official contexts and is not the diminutive of "Michael"), nor his career described in the Byzantine sources allow an identification with Michael, the son of the Rus' prince and Hungarian Ban Rostislav Mikhailovich, nor any other member of the house of Chernigov.

Although he might have acquired some support in the capital Tarnovo and possibly in Preslav, he was faced by the hostility of much of the provincial nobility.

[7] Following an obscure unsuccessful campaign against Theodore II Lascaris of the Empire of Nicaea, Mitso lost control even over the commoners.

In exchange for asylum and lands, he turned over Mesembria and its environs to Emperor Michael VIII Palaiologos in 1261[9] or 1263[10] and sought refuge in Nicaea.

The date of his death is unknown, but it is likely that he was no longer alive in 1277/1278, when his son Ivan Asen III was put forward as a claimant to the Bulgarian throne by the Byzantine Emperor Michael VIII.