Immediately upon her entering the House of Commons Castle was appointed Parliamentary Private Secretary (PPS) to Sir Stafford Cripps, President of the Board of Trade, who had known her as a member of the pre-war Socialist League. Harold Wilson succeeded Cripps in 1947 and retained Castle as his PPS, marking the beginning of the pair's lengthy political relationship. She gained further experience as the UK's alternate delegate to the United Nations General Assembly for 1949–1950, when she displayed particular concerned for social and humanitarian issues. She soon achieved a reputation as a left-winger and a rousing speaker. During the 1950s she was a high-profile Bevanite, and made a name for herself as a vocal advocate of decolonisation and the Anti-Apartheid Movement.