She will lead the commission's press team in the new role of head of news.
She joins from Citizens Advice, where she had been head of media since June last year. Before that, Miller was senior press officer at the Salvation Army.
"This is a key time for the commission as we take on our new responsibilities under the Charities Act and continue to develop our role in supporting and regulating the sector," said Miller. "The current debate about the campaigning role of charities provides us with an opportunity to affirm the vital contribution of the charity sector to society and the commission's support for charity campaigning."