Brownlee joins AOP with more than 10 years' experience in digital media, having worked at women's portal Handbag.com, Yahoo! and having developed online partnerships at Channel 4.
She replaces White, who left earlier this year to take up a job at the BBC Worldwide's digital delivery team as new-launch director working across all aspects of the UK web industry.
Simon Waldman, AOP chairman and group director of digital strategy and development at Guardian Media Group, said: "Ruth is a natural choice to be AOP's new director.
"Her industry experience and breadth of online expertise are second to none, and she will take the association to the next level, in what is an exciting period of growth for online publishing across the board."
Brownlee said: "I am immensely excited about the role and the opportunity to lead an organisation that has such an impressive set of publishers behind it. There is real ambition within AOP and that excites me."
Brownlee has most recently been working as a digital strategy adviser to a variety of top publishing brands. She will take up her position with AOP at the end of January.
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