Lamont served in successive governments under Margaret Thatcher and John Major for a total of 14 years, in the Departments of Energy, Industry, Defence and the Treasury. In 1986, he moved to the Treasury, first as Financial Secretary to the Treasury, then Chief Secretary to the Treasury (succeeding John Major in the latter job on Major's promotion to Foreign Secretary in July 1989) under Chancellor Nigel Lawson, whom he tried unsuccessfully to persuade not to resign from the government on the morning of 26 October 1989 – Lawson resigned that evening. Lamont remained as Chief Secretary to the Treasury under Major's Chancellorship. In this position he acquiesced in Major's decision to join the European Exchange Rate Mechanism (ERM) at a central parity of 2.95 Deutschmarks to the Pound, although neither he or any other Cabinet ministers were involved or informed about the decision before it had been made.