When his son, Sab Than, wanted to marry Dejah Thoris, Than Kosis agreed to have them married. But Dejah Thoris did not love Sab Than and only wanted to marry him so that there would be peace between Helium and Zodanga. Dejah Thoris had also believed at the time her true lover, John Carter to be dead.
When John Carter crashed the wedding of Sab Than and Dejah Thoris, Than Kosis fought back against Carter and the Tharkian warriors that were invading Zodanga. However, Than Kosis was killed along with his son.