As a result, at the age of 18, Cook moved to Porth in South Wales, and later to Merthyr Tydfil, to find work in the coal mines; and was also a lay preacher in the local Baptist chapel.

In this mining town, Cook first became involved in politics, committing active service to the Independent Labour Party (ILP).

In September 1920, Cook was a founding member of the short-lived Communist Party of South Wales and the West of England, which had emerged from South Wales Socialist Society sharing a similar rejection of parliamentarism as the Communist Party (British Section of the Third International) who held their third conference in Cardiff in December 1920.

In 1921, Cook was selected to be the SWMF representative on the MFGB executive and spent another brief period in prison in the same year for incitement and unlawful assembly.

Arthur Horner, a leading South Wales Communist and mining militant described Cook's tenure as General Secretary as "a time for new ideas — an agitator, a man with a sense of adventure".