Born in Maesteg, Cox grew up in Cwmfelin, where he worked in a coal mine from a young age.
[2] His family was highly religious, and he attended chapel three times each Sunday, but he lost interest in religion after becoming active in the trade union movement, and was involved in the South Wales miners' strike of 1915.
[3] He was elected to the management committee of the Garth Miners' Institute at the age of 18 and, in 1920, he became the lodge's delegate to coalfield conferences.
[2] In 1923, the South Wales Miners' Federation granted Cox a scholarship, enabling him to study at the Central Labour College in London and, after the 1924 general election, he joined the Communist Party of Great Britain (CPGB).
[5] Later that year, Cox became secretary of the International Department of the CPGB, and was heavily involved in the Movement for Colonial Freedom.