The development came just days after the Duke of Sussex was named on a board of 15 members of the Commission of Misinformation Disorder in the US, where he will target the "avalanche of misinformation" online which he claims has become a "humanitarian issue".
Mills said she made the decision to step down once it was clear no apology was coming from the Society of Editors.
"That apology and clear statement about the society's current view in regards to whether the lack of diversity in our newsrooms amounts to structural racism in the UK media is still not forthcoming," she said, via the Guardian.
"I have lost confidence in the Society of Editors ever delivering such a statement."
Mills claimed print media was especially behind in terms of diversity in the newsroom and lamented the fact there was "no agreement on the scale of the problem".
"Often segments of our media do operate a double standard when it comes to race and I believe it is the duty of the Society of Editors to come out and say so," she said. "If there is no agreement even on the scale of the problem we are dealing with then it is hard to have confidence in the decisions of the board going forward to remedy it.
"Unlike many board members I have no institutional ties keeping me from resigning on this matter of principle, so today that is what I am doing."