Brownlee will leave at the end of October, having joined the AOP in December last year when she replaced Alex White, who left to join BBC Worldwide.
She has more than 10 years' experience in digital media, having worked at women's portal , and at Channel 4 where she developed online partnerships.
Brownlee said: "I've had to make the very tough decision to resign from my position at AOP - based purely on the needs of my young family.
"I've totally enjoyed my time here: it's been a challenging, stimulating and immensely satisfying job and one that I will find hard to leave. I've worked with an inspiring team, and I know the AOP has a very exciting future.
"Its membership and ambition is strong - with some great plans afoot for '09 - I'll miss being part of it."
The AOP said it had started the search for a successor.
Simon Waldman, AOP chairman, said: "Ruth has given us eight phenomenally productive months, she has given the AOP some much needed energy and clarity of purpose and she will leave us in a much better shape to drive the organisation forward and with a greater vision of what AOP means to its members, partners and stakeholders."