His two roles at the top of government have in recent decades been filled by the same individual. As head of the civil service, Lord Turnbull was akin to the chief executive of the organisation, though the lines of reporting are somewhat more complex than is typical in the private sector since Permanent Secretaries (senior civil servants within each department of government) report to ministers. As Cabinet Secretary, a post created in 1916, Turnbull was responsible for the organisation of the Cabinet Office, providing support to the Prime Minister and to the government as a whole. When Turnbull succeeded to the dual role on 2 September 2002, Prime Minister Tony Blair asked him to focus on the management of the civil service, and to make its reorganisation his priority.
Turnbull became involved in controversy when on 28 February2004 he wrote a formal letter admonishing ex-minister Clare Short for making media statements alleging that British intelligence had intercepted communications from (amongst others) Secretary General of the United Nations Kofi Annan. Short made the confidential letter public, and in turn rebuked Turnbull for allegedly allowing the government decision-making machinery to crumble in the run-up to the 2003 Iraq war. Short suggested that the government's legal expert, Attorney General Lord Goldsmith, had been "leant on" to provide advice that war would be legal. She argued that Turnbull had been responsible for what she alleged was inadequate Cabinet scrutiny of the legal advice, of the basis for the decision to go to war and the alternatives:
In March 2005, Lord Turnbull revealed that Lord Goldsmith's opinion on the legality of the Iraq War was only one page long.
On 20 March 2007, the day before the 2007 budget was announced, he gave an interview with the Financial Times in which he described Gordon Brown as acting with "Stalinist ruthlessness". He is acting as an advisor to Booz Allen Hamilton.
His previous positions included Principal Private Secretary to the Prime Minister (1988-1992) and Permanent Secretary to the Treasury (1998-2002), the latter traditionally the second-highest-ranking Civil Service post.