Brownlee, who took over at the AOP in December following predecessor Alex White's move to BBC Worldwide, has announced she will be leaving the association at the end of next month.
The mother of two pre-school children said the decision had been "an extremely tough one to make," adding that the association had many exciting plans for the coming months, starting with its annual conference in October.
It is understood Brownlee initially signed up for a three-day role when she joined the association, but the continued growth of the digital body, coupled with its burgeoning activities, has demanded a full-time position.
She confirmed the AOP, chaired by the Guardian's Simon Waldman, will be looking to recruit a full-time successor.
Waldman 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."
Borne out of a collaboration of traditional content publishers in 2002, and initially led by the magazine body PPA, the AOP now represents more than 160 content providers, encompassing newspaper and magazine publishing, TV and radio broadcasting and pure online media.
Separately, the AOP has appointed ITV’s Gary Cole as chairman of its new commercial working group which aims to to ensure that "quality, branded content sites" are well placed to take advantage of growth in the digital market.
Alex White
Brownlee to leave AOP due to family commitments
LONDON - Ruth Brownlee is set to step down as director of the Association of Online Publishers (AOP) after just 10 months, to spend more time with her family.