NEW YORK (CNN) - Shaken by the criminal conviction and sentencing of Martha Stewart, the company she founded announced Tuesday it is appointing a new chairman, Thomas C. Siekman, who has served as a director of Martha Stewart Living Omnimedia.

He succeeds Jeffrey W. Ubben, who remains head of ValueAct Capital, an investment firm that is the company's second-largest shareholder.

"With the company having weathered a most difficult period associated with Martha Stewart's personal legal situation, I felt this was the right time to turn over the chairmanship to someone who has the skills and, most importantly, sufficient time available to guide the board and the company forward from this point," Ubben said.

Stewart's legal troubles forced her to step down as chairman, chief executive officer and director. Stewart was sentenced July 16 to five months in prison and five months of home detention after a jury convicted her of lying to federal investigators about her sale of ImClone stock.

Martha Stewart Living Omnimedia is suffering as a result of its founder's conviction. Advertising revenue in the company's flagship magazine, Martha Stewart Living, plummeted 52 percent during the first half of the year.

Siekman, a director at MSO since last August, is an attorney with Skadden, Arps, Slate, Meagher & Flom and former general counsel of Compaq Computer and Digital Equipment.

Martha Stewart Living Omnimedia also announced Arthur C. Martinez, former chairman and CEO of Sears Roebuck & Co., has stepped down from the board of directors and his position as lead director.

Entertainment mogul Charles Koppleman, former chief of EMI Records, will fill the open director slot, the company said.