After a brief reign, his brother Pir Ahmet Bey lost most his beylik to the Ottoman Empire, and both brothers escaped to AkkoyunluTurkmen's territory. With Akkoyunlu support they tried to regain their former possessions. Although they weren't able regain their losses, Kasım allied himself with Republic of Venice, and with the help of Venetian navy, he was briefly able to keep Silifke and the Mediterranean coast of the beylik. However, following the campaigns of Gedik Ahmed Pasha of the Ottoman Empire, he lost all of his possessions in 1475.
Nevertheless, during the Ottoman Interregnum following the death of the Ottoman sultan Mehmed II, Kasım allied himself with the pretender Cem and almost reconstructed the former Karaman beylik. After Cem escaped and took refuge with the Knights of Rhodes, however, Kasım acknowledged Ottoman suzerainty. He continued as an Ottoman governor until his death in 1493.