Constantine Tih mounted the Bulgarian throne after the death of Michael II Asen, but the circumstances of his ascension are obscure. Michael Asen was murdered by his cousin, Kaliman in late 1256 or early 1257. Before long, Kaliman was also killed, and the male line of the Asen dynasty died out. 

Rostislav Mikhailovich, Duke of Macsó (who was Michael and Kaliman's father-in-law), and the boyar Mitso (who was Michael's brother-in-law), laid claim to Bulgaria. Rostislav captured Vidin, Mitso held sway over southeastern Bulgaria, but none of them could secure the support of the boyars who controlled Tarnovo. The latter offered the throne Constantine who accepted the election. 

Constantine divorced his first wife, and married Irene Doukaina Laskarina in 1258. Irene was the daughter of Theodore II Laskaris, Emperor of Nicaea, and Elena of Bulgaria, a daughter of Ivan Asen II of Bulgaria. The marriage with a descendant of the Bulgarian royal family strengthened his position. He was thereafter called Konstantin Asen. The marriage also forged an alliance between Bulgaria and Nicaea, which was confirmed one or two years later, when the Byzantine historian and official George Akropolites came to Tarnovo.