Clarken will be based in Nielsen Online's regional headquarters in Sydney, Australia and will lead the company's commercial business in Australia and New Zealand.
As part of Nielsen Online's Asia-Pacific management team, she will report to Jonathan Carson, president international at Nielsen Online.
Carson said: "Megan's understanding of client needs, her strong leadership skills and familiarity with the Pacific market make her perfectly suited for this role. Nielsen Online clients and the internet industry in the region will truly benefit from her experience."
Clarken joined Nielsen Online, then called Nielsen//NetRatings, in 2004 as vice-president of web analytics. She was based in Sydney and managed the company's web analytics product portfolio.
She relocated to New York in 2005 as vice-president of web analytics and program, with responsibility for the strategic direction of the company's overall product development.
Her previous positions include senior roles with News Ltd, Akamai, Peakhour and ninemsn.
She said: "We have a terrific organisation in the Pacific region and they service a growing and dynamic marketplace.
"I look forward to returning to Sydney and to working with the team to deliver innovative products, outstanding service and thought leadership to our clients and the industry."