Williams joined the Labour Party after he moved to Cambridge in 1961, but resigned in 1966 after the new majority Labour government had broken the seafarers' strike and introduced public expenditure cuts. He joined the Vietnam Solidarity Campaign, and wrote the May Day Manifesto (published 1967) along with Edward Thompson and Stuart Hall. It is claimed that Williams later became a Plaid Cymru member and a Welsh nationalist. He retired from Cambridge in 1983 and spent his last years in Saffron Walden. While there he wrote "Loyalties", a novel about a fictional group of upper-class radicals attracted to 1930s Communism.