A graduate of the University of Oxford, Castle worked as a journalist for both "Tribune" and the "Daily Mirror", before being elected to Parliament as MP for Blackburn at the 1945 election. During the Attlee Government, she was Parliamentary Private Secretary to Stafford Cripps, and later to Harold Wilson, marking the beginning of their partnership. She was a strong supporter of Wilson during his campaign to become Leader of the Labour Party, and following his victory at the 1964 election, Wilson appointed Castle to the Cabinet as Minister for Overseas Development, and later as Minister of Transport. In the latter role, she proved an effective reformer, overseeing the introduction of permanent speed limits for the first time on British roads, as well as legislating for breathalyser tests and compulsory seat belts.