Martin, who is head of philanthropic partnerships at GNM, takes up the post on 29 August. Prior to GNM, she worked at anti-poverty group ONE.org, as a freelancer, and in the civil service, latterly as chief press officer in the Prime Minister's Office between 2008 and 2010.
In what is a new role for the charity, she reports to director of policy and advocacy James Plunkett, who had also worked in Downing Street between 2008 and 2009.
She said: "As increasing numbers of people struggle to cope with insecure work, changes to benefits, and rising household bills – as well as the uncertainty of Brexit - the support provided by Citizens Advice has never been more important.
"I’m thrilled to be joining the team at this crucial time, as we work to ensure that policymakers fully understand the effect of their decisions on people’s lives – while also modernising the way we deliver our own services so we can help people solve the problems that are holding them back."
Citizens Advice is a national charity that undertakes policy and advocacy work, and operates in collaboration with a network of 300 independent local services which provide advice face-to-face, on the phone or online.
Recent campaign successes have included securing a reduction in energy bills for vulnerable customers, improved deposit protection for private renters and tighter controls on logbook loans.