He joined Breiz Atao in 1919 and became president of "Unvaniez Yaouankiz Vreiz" ("Youth Union of Brittany") in 1922. Together with Roparz Hemon, he created the literary magazine "Gwalarn" (1925), and was included in the Breton delegation to the First Pan-Celtic Congress in Dublin (alongside François Jaffrennou, Morvan Marchal, and Yves Le Drézen). Subsequently, Mordrel became co-president of the Breton Autonomist Party (Parti Autonomiste Breton, or PAB), and then its secretary for propaganda. During the same year, he started mixing his political and aesthetical ideals, adapting Art Deco to Breton themes, and aligning himself with the Breton art movement Seiz Breur.