The agency, which started in July and has Public Health Wales as one of its clients, signed contracts with Comic Relief on Thursday to begin work.
Cowshed’s activities will cover Wales, the West, the South West and West Midlands as part of an overall UK media team controlled in-house by the charity.
Cowshed’s brief will involve media relations across its areas of geographical responsibility as well as managing stakeholder relations with the BBC.
It will also devise creative stunts and activities to drive people towards taking part in Red Nose Day 2015.
Vicki Spencer-Francis, founder and managing director of Cowshed, said: "At Cowshed we want to use our experience in fundraising, communication, marketing and campaigning to work with organisations across the public, private and voluntary sectors for one simple reason; to make a difference."