Mitchell – who is set to join the PR agency next month – will take up the role of senior account director while deputy head of public affairs Langley assumes the role of acting head of public affairs.
Mitchell worked on public affairs strategy at the London Borough of Newham in the run-up to the 2012 Olympic and Paralympic Games and as a councillor in Hackney until May 2014.
Before this, Mitchell built the Charity Retail Association's public affairs team, growing expertise in business rates, high street and retail policy.
Mitchell said: "Working alongside the only UK PA team to have won two Gold Cannes Lions is an absolute honour and I can’t wait to work with the best and brightest in the industry."
Langley added: "With the arrival of Wendy, our team goes from strength to strength. Wendy has an excellent reputation as a public affairs practitioner, and brings further experience in retail, transport, local government and sport."