After training in secretarial skills, Rantzen was recruited by BBC Radio as a trainee studio manager. She began her television career as a clerk in the programme planning department, then obtained her first production job working as a researcher on the BBC One late-night satire programme BBC3 (1965–66). Having worked as a researcher on a number of current affairs programmes, she moved to the award-winning BBC Two documentary series Man Alive in the mid-1960s.