During his youth, he fled for unknown reasons from his brother Shahrivash, and began serving the Bavandid ruler Shah Ghazi Rustam IV.
Kai Ka'us quickly rose into high ranks, and after the death of Shah Ghazi Rustam's vassal Kiya Buzurg in 1156, he was appointed as the successor of the latter.
However, after having gained control of his family's domains, Kai Ka'us, along with a local ruler named Fakhr al-Dawla Garshasp, revolted against Shah Ghazi Rustam's successor Hasan I. Hasan I shortly sent an army against the latter, which was, however, defeated by Kai Ka'us.
Sometime later, Kai Ka'us' son Justan died, and only his 1-year-old grandson Zarrinkamar survived, whom Ardashir I agreed to raise and make ruler of the Paduspanid dynasty when the latter had grown up.
After his death, the nobles of the Paduspanid capital of Ruyan made his nephew Hazarasp II the new ruler of the dynasty.