He lost his place on the BBC executive committee when it was reduced in size this year.
Mr Purnell, who served as culture secretary under New Labour before moving to the BBC, announced he was taking up a role at the University of the Arts London.
He lost his place on the BBC executive committee when it was reduced in size this year, by new chief executive Tim Davie.
Mr Purnell, paid £315,000 by the BBC, has now announced he is leaving the BBC and has been appointed president and vice-chancellor of the University of the Arts London.
Mr Davie said Mr Purnell “has been a superb member of the senior team at the BBC.
“He has many major achievements, from his role in negotiating the current Charter, to his transformation of our network radio services and making BBC Sounds the success it is today”.
Mr Davie added: “For me personally, James has always been a source of insightful advice and a great colleague.”
Mr Purnell, who did not apply for the role of BBC director-general after Tony Hall stepped down, wrote in an email to staff in BBC radio, music and arts that he is “proud of what we’ve done together in the last four years”.
And he said in a statement from the University of the Arts London: “I couldn’t be happier to join the brilliant team at UAL and I look forward to working with students and staff alike across all colleges. ”
Mr Purnell first worked at the BBC in the 1990s as head of corporate planning, but left to become a special adviser to Tony Blair after he became prime minister.
He resigned from the government in June 2009, criticising the leadership of Gordon Brown.
Previous positions at the BBC include director of strategy and digital and his role has included the development of the corporation’s education strategy, such as Bitesize.