Whitlock was elected as the Member of Parliament (MP) for Nottingham North in 1959. Throughout his career, he was a champion of improved conditions for office workers. A party whip from 1962, Whitlock was appointed Vice-Chamberlain of the Household in 1964, and retained the office until 1966. He was then briefly a Lord Commissioner of the Treasury before being appointed Comptroller of the Household. In 1967, he was again briefly a Commissioner of the Treasury before being appointed Under-Secretary of State for Commonwealth Relations. At that office, he was responsible for African affairs, and he advocated the admission of Asians expelled from Uganda into Britain. At the merger of the Foreign Office and Commonwealth Relations in 1968, he became Parliamentary Under-Secretary of State for Foreign and Commonwealth Affairs. Sent in 1969 to negotiate with the fledgling government of Anguilla, then seceding from Saint Kitts and Nevis, he was unceremoniously expelled from the country at gunpoint. The incident ended his ministerial career.