Llamas, who became Nielsen's HR chief in June 2007, presided over a period of significant downsizing, as the private equity owned company sought to improve its margins and operating efficiencies in preparation to take itself public.
Since 2007, thousands of Nielsen employees have been let go, and much of its technology has been outsourced to overseas, third-party providers.
During that period, Nielsen said Llamas helped globalize Nielsen's workforce and introduced a "contemporary leadership development process and commitment across the company."
Nielsen said Llamas informed the company of his desire to move on "early in 2011."
Llamas joined Nielsen from The Cleveland Clinic, where he was chief administrative officer.