Ms. Karo was most recently president of integrated accounts at Omnicom Media Group and lead on its Apple account.
In a statement, Omnicom did not explain the reasons for the change at the top but noted that her appointment is effective immediately.
But executives familiar with the global media agency said that Mr. McLean's management style at times didn't fit with its culture. Mr. McLean was hired as U.S. CEO in December 2010 after serving as worldwide chief-business development officer at WPP's Group M.
Mr. McLean declined to comment.
Ms. Karo will retain many of her OMG corporate duties and continue to work with clients within the Media Arts Lab at sibling shop OMD. She's based in Los Angeles but will now spend more time in New York.
Ms. Karo has a long history with Apple, a key Omnicom client. In 1999, Ad Age reported that when she was the corporate media director for TBWA/Chiat/Day Steve Jobs said, "She's incredibly talented and a key member of the Apple advertising team."
She left the ad firm in 2002 to join OMD, Los Angeles. In May 2008 she was promoted to president of integrated accounts for OMG, the umbrella over media giants OMD and PHD.
Omnicom announced additional changes. Steve Williams, CEO of OMD Group UK, will be named president of PHD New York. PHD's current president and chief digital officer, Craig Atkinson, will move into the chief operating seat for PHD U.S. while retaining his CDO role. Both Mr. Williams and Mr. Atkinson will report directly to Ms. Karo.