Karmazin said he was quitting Viacom in June this year after reports of tension between him and Viacom's chairman and chief executive Sumner Redstone.
At Sirius, he takes over from Joseph Clayton, who will remain at the company as chairman.
This second high-profile appointment is a coup for Sirius, which is making a bold move to put subscription satellite radio on the map. Its five-year, multimillion-dollar deal with Stern, who is hugely popular in the US, boosted the company's share price and gained it wide coverage in the media when it was revealed in October.
Stern has been using his instinct for drawing publicity to both antagonise his current employer, Viacom's Infinity Broadcasting, as well as plugging the medium he will soon be working for.
Just this week, Stern drew around a thousand fans to New York's Union Square, when he announced on air that he would be giving away 500 satellite radios and subscriptions to Sirius, which broadcasts commercial-free music and sports programming via satellite to homes and cars across the US.
Stern is looking forward to the freedom that broadcasting on satellite radio will give him. In the new climate of fear among US broadcasters, Stern has been widely criticised for talking about sex on his programme. In response he has dropped the smut and instead rails about censorship and freedom of speech.
Karmazin was at Viacom, which owns television network CBS as well as MTV and VH1, for 20 years, before quitting in June. He was replaced by two executives, Leslie Moonves and Tom Freston.
"This is a perfect opportunity for me because I want to lead a growth company that can reshape the landscape of the radio business. I took Inifinity Broadcasting and Westwood One to leadership positions in the industry and am confident that Sirius will become a market leader in short order," Karmazin said.
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