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Mellor became a barrister in 1972, and a Queen's Counsel in 1987 but is currently not practising. He became a Member of Parliament (MP) representing the London constituency of Putney, at the 1979 General Election. In 1990, John Major made him Chief Secretary to the Treasury but after a sex scandal in 1992, Mellor was forced to resign and lost his seat to the Labour Party in the 1997 General Election.