Mills joins the Anglo-Australian mining firm after ten years heading media relations at various major UK banks.
She takes on the new role of global head of media practice, based in London and reporting to global head of comms and external relations Andrew Vickerman.
The London-listed company has come under heavy scrutiny in recent years, over issues including the environment and human rights.
The company's involvement in Papua New Guinea in the 1990s was especially controversial, and it has since worked to clean up its tainted human-rights image.
Mills was hired via headhunter Watson Helsby and joins from Royal Bank of Scotland, where she has spent six months as interim head of media relations for retail banking.
She was previously head of media relations at Abbey National and head of retail media relations at NatWest.
Mills said: ‘Rio Tinto has done some excellent local media relations work where its mines are situated. We now want to raise our game and I am putting together a corporate media team based in the UK, Australia and the Americas.
She added: ‘I aim to create a network of local media relations people.'
Rio Tinto posted record full-year profits last month, driven by surging commodity prices and strong demand from China. Its net profits for 2006 were £5.74bn.