After an early career in the financial sector, Fleming joined MI5 in 1993. He gained extensive operational, investigative and leadership experience across the full range of national security work. He helped shape MI5's response to the London terrorist attacks in 2005, led the revision and publication of the Government's counter-terrorism strategy, CONTEST and was promoted to Assistant Director General to lead MI5's preparations for the London 2012 Olympics. He then spent four years as Deputy Director General with responsibility for all investigations and operations.