Mark Davies has spent the past five years as special adviser to ex-justice secretary Straw. He also worked for Valerie Amos, when she was leader of the House of Lords. Before that he was a journalist.
He replaces Paul Corry, who was director of public affairs at the charity. Corry left the organisation earlier this year.
Davies, who joined Rethink last week, said: 'The stigma and prejudice that still exists in relation to mental illness is one of the biggest challenges currently facing our society.'
Rethink is calling for better, more cost-effective treatment for people affected by mental illness.
A poll commissioned by the charity revealed that only one in six people with mental illness were receiving all the treatments recommended to them.