Beck will have a role in developing Nationwide's strategic direction and will also be responsible for all aspects of external relations planning, including investor relations, media engagement, public affairs and company narrative.
On 21 November he will take up the newly created role of director of strategic planning and external relations, sitting on the firm's executive committee alongside Garner, who joined the bank in April.
Garner said that it was a natural fit to combine the firm's strategy and stakeholder relations functions, given the business' building society status.
He said: "As a mutual building society, we have the privilege of being owned by our members, which allows us to operate in the wider interests of members, customers and society. It puts a special premium on understanding and engaging our stakeholders and aligning interests with our society strategy."
Beck is currently a partner at Teneo, a corporate advisory group founded in the US 2011, which Beck established in London in 2012. It has since acquired several UK businesses including Blue Rubicon, before restructuring and rebranding as Teneo Blue Rubicon in September.
Prior to this, he spent 13 years at HSBC, latterly as the bank's global comms chief.
He said: "Nationwide’s team has done a tremendous job building an enviable reputation in a sector that makes such an important contribution to society, but has been dogged by disaster and difficulty. We have a golden opportunity to demonstrate what being a member-owned organisation means, and to take a position of real leadership in the industry."
Beck will also "continue in a private capacity to advise several CEOs and chairmen in the UK and overseas in non-competing sectors", a statement from Nationwide said.
Nationwide also works with the agencies Brunswick and Lansons.