The Mark Penn era at Burson-Marsteller is over.

After six years as CEO of the PR firm, Penn will join Microsoft—a longtime client of the WPP-owned Burson-Marsteller (B-M)—as vice president of strategic and special projects. He’ll report to Microsoft CEO Steve Ballmer.

“Mark has an incredible background in research, demographics, marketing and positioning and a proven history in developing unique insights that drive success,” Ballmer said in a press release. “With a strong set of products and an exciting pipeline for the next year, Mark’s experience and out-of-the-box thinking will help us more effectively reach new consumers and grow market share.”

Penn was named CEO of B-M in 2006. At the time, B-M had shuffled through three chief executives in five years and experienced a decline in fee income, according to The Holmes Report. Under Penn’s watch, the firm’s fee income is expected to reach nearly $500 million this year.

He also founded the research firm Penn Schoen Berland and served as campaign advisor to Bill and Hillary Clinton. Penn resigned from the latter Clinton’s presidential campaign in April 2008 after it was reported that he met with Colombian ambassadors to discuss a trade pact that Clinton opposed.

Donald A. Baer will replace Penn as chief executive at B-M. He joined the firm four years ago as worldwide vice chairman and chief strategy officer. Prior to B-M, Baer worked for Discovery Communications and in key advisor and communication positions for Bill Clinton’s campaign for president and during his presidency. He is also a former journalist and media attorney.

“Burson-Marsteller continues to be an industry leader with strategic public relations capabilities and a global footprint that few firms can match,” said Sir Martin Sorrell, CEO of WPP, said in a press release. “I am confident the firm’s track record of success and its reputation for delivering results for its clients will continue under Don Baer’s leadership.”

This morning, WPP shares were down slightly, while Microsoft’s were up by about a half percent.

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