Frohlich joined Ogilvy in 2012 and became chief operating officer for the EMEA region in February. He will continue to act as UK CEO for the company in addition to his new EMEA duties.
Smith, who joined Ogilvy in 2010 to run the EMEA region, was promoted to global CEO last year but retained the title of EMEA CEO until today.
Earlier this month the firm announced it had trebled the membership of its executive committee.
Smith said: "Michael is simply one of the best PR people I have had the pleasure to hire and work with. Through his energy and passion, he has tirelessly built Ogilvy PR UK into the successful, award-winning agency we now are.
"Under his leadership we have transformed into a modern, earned media operation that is a highly creative, social-by-design with content totally at the center of our offer. This announcement is one in a series made recently to show how we will structure Ogilvy PR for the future."
Frohlich, whose first job will be to create a five-year strategy for Ogilvy PR in the EMEA region, said: "I thank Stuart for the successful partnership we enjoy and am excited to embrace the opportunity to continue growing our people, brand and business in the EMEA region at the service of our clients."