Karmazin had said earlier this year that he would leave because of the pending Liberty takeover.
Liberty already has the equivalent of nearly 48 percent of Sirius XM, though the takeover still has to be approved by federal regulators.
Liberty rescued Sirius XM from near-bankruptcy in February 2009 by lending it some $530 million in exchange for a 40 percent stake.
Since then, Sirius XM has stabilized. As of the end of June it had more than 22 million subscribers.
—AP contributed to this report.