Stewart was born in Mirfield, Yorkshire, England, the son of Gladys (née Barrowclough), a weaver and textile worker, and Alfred Stewart, a Regimental Sergeant Major in the British Army who served with the King's Own Yorkshire Light Infantry and previously worked as a general labourer and as a postman. Throughout childhood, he endured great poverty and disadvantage, an experience which influenced his later political and ideological beliefs. In 2006 Stewart made a short video against violence for Amnesty International, in which he recollected his father's physical attacks on his mother and the effect it had on him as a child. He attended Crowlees C of E Junior and Infants School, and in 1951, aged 11, he entered Mirfield Secondary Modern School, where he continued to study drama.

At age 15, Stewart dropped out of school and increased his participation in local theatre. He acquired a job as a newspaper reporter and obituary writer, but after a year, his employer gave him an ulti