Cowshed was appointed to work on the accounts following a competitive pitch.
For the Green Man music festival Cowshed will work alongside London-based PR agency, Bleached.
Cowshed will also manage all PR and marketing support for the largest food festival in Wales, The Abergavenny Food Festival, and boutique family festival, The Big Day Out at Humble by Nature. The agency, which was set up last year, currently works for clients including Comic Relief, Stoptober, Public Health Wales and Brace’s Bakery.
Fiona Stewart, founder of Green Man, said: "It's exciting to be working with another new and forward-thinking Welsh business. Green Man 2015 promises to be one of our strongest years as we bring the return of the Super Furry Animals, St Vincent and Hot Chip to the Black Mountains."
Cowshed founder Vicki Spencer-Francis added: ‘We are just about to celebrate our first birthday and have been overwhelmed by the support shown by everyone we have worked with so far. Working with two of the biggest events in Wales within our first year is just incredible and beyond our expectations."