Sarah Mowatt joins from software firm Macromedia Europe, where she was European comms manager. She will oversee a small number of Lewis staff, working on pro bono projects, who will be picked from across the agency.
She will also advise clients of Lewis sister company Octane, including BT, Bonusprint and Voxgen.
Mowatt spent eight years at Macromedia Europe, where she led charity campaigns, including Comic Relief and The Prince's Trust. Her role was made redundant earlier this year following the company's acquisition by rival Adobe in December. Lewis's pro bono accounts include Byte Night, London Cares Day and The Living Rainforest.
Mowatt will also oversee the company's charity staff support scheme, which gives staff two days' annual leave to carry out charity work.