In her 18 years in Omnicom media roles, Monica Karo has contributed to the growth of important client brands like Apple. Now, in her latest role as CEO of PHD U.S., she has the opportunity to do as much for an agency brand as she did for those clients.

Ms. Karo became media director at TBWAChiatDay North America in 1994. When OMD launched in 2002, she was appointed as managing director, Los Angeles; and in 2009 she took a global role as president of integrated accounts at Omnicom Media Group.

Now, she'll continue to build a consistent global presence for PHD, aiming to expand client relationships in new markets.

"There are going to be more and more global opportunities," she said. "We can't have disconnects between PHD U.S. and the rest of the globe."

She'll also hold on to the relationship with Apple at sibling shop OMD. She recalled: 15 years ago, Steve Jobs returned to Apple, and the next thing Ms. Karo knew she was meeting with his team at Apple headquarters every Wednesday. She'll continue to strengthen those ties while facing new challenges.

"As we grow, I want to maintain the PHD personality and approach," Ms. Karo said. "It's a tricky balance, not losing that as you get larger."

What is your favorite book?

A toss up between "The Fountainhead" and "Anna Karenina." But favorite recent book is "Start-Up Nation" by Dan Senor and Saul Singer.

What's one fact about yourself that will surprise people?

I have run the New York Marathon several times, and my best time was three hours, 14 minutes, 21 seconds.

If I wasn't doing this job, I'd be ...

In something related to education.

What's your favorite kind of music?

World music.

What is your dream vacation spot?

Anywhere with my husband and two children.