Paynter was born in Cardiff, where he had a basic education before going to work at a colliery at the age of fourteen. By the age of eighteen, he was working on the coal-face, and soon joined the Communist Party. He was instrumental in setting up the National Unemployed Workers' Movement, and in 1937 he joined the British Battalion of the International Brigades to fight in the Spanish Civil War. In 1951 he became President of the South Wales Miners' Federation, and from 1959 to 1969 he was General Secretary of the National Union of Mineworkers. He was also a member of Acas.