In 1975, Brennan began work with the Department of Health and Social Security. She worked several positions in the health division before joining the social security division, where she worked on disability benefits policy from 1990 to 1993. Brennan moved into operations, becoming director of the department's IT Services Agency from 1993 to 1995. From 1995 to 1997, she was disability policy director at the Department of Social Security. After that she became director of Change Management at the department's Benefits Agency, where she oversaw a staff of more than 1,000 employees. She was appointed director-general of the Department for Work and Pensions in 1999.
Department for Environment, Food and Rural Affairs
From 2004 to 2006, Brennan served as the director-general of the Living Land and Seas division at the Department for Environment, Food and Rural Affairs. Her responsibilities included leadership of the protection of wildlife and the countryside, and leadership of the division's strategy for rural disadvantage. She led the merging and creation of two new non-departmental public bodies, the restructuring delivery of grants and advice to rural business, and the implementation of a review of rural delivery.
Ministry of Justice
In 2006 and 2007, Brennan was chief executive of the Office for Criminal Justice Reform, a now defunct office that supported joint cooperation among the three criminal justice departments and criminal justice agencies. Brennan led the review to create a new structure for the Ministry of Justice, where she worked from April to September 2008 as director-general of Corporate Performance. Her area of responsibility encompassed leadership of the corporate functions in the Ministry, including finance, IT, human resources, strategy and planning, communications, and research and analysis.
Ministry of Defence
In October 2008, Brennan joined the Ministry of Defence as Second Permanent Under-Secretary. She was appointed Permanent Secretary in October 2010, replacing Sir William Jeffrey. As Permanent Secretary, Brennan's responsibilities included acting as joint chief operating officer of the department, responsible for organisation and management, along with Chief of the Defence. She was also principal accounting officer for the Department of Defence, and as such was accountable to Parliament for the efficient and effective use of department resources. During her time at the Ministry of Defence, Brennan made headlines when she stated in 2011 that the British military needed to do more to integrate women into the higher ranks, saying, "They need toget on with it, and promote some women... are not going to hit the nuclear trigger button by mistake." She also oversaw the 2011 investigation into the conduct of then Minister of DefenceLiam Fox, who had allowed his friend Adam Werritty to join him on foreign missions, attend official diplomatic meetings and, despite having no security clearance, visit him repeatedly at the Ministry office.
Return to Ministry of Justice
On 12 June 2012, it was announced by Sir Bob Kerslake, Head of Her Majesty's Civil Service, that Brennan would return to the Ministry of Justice as Permanent Secretary and Clerk of the Crown in Chancery. She replaced Sir Suma Chakrabarti, who left to become president of the European Bank for Reconstruction and Development. In an interview in July 2012, Brennan spoke again about the need for gender equity in the workplace, saying, "The representation of women at the top levels of the civil service has really changed, but I've worked in departments where even when we got to several women on the board, it just takes a couple of departures and it's gone. You really need a relentless focus as you can think you have won the battle and then you find it's too easy to slip back." Brennan retired from the civil service at the end of July 2015. She was succeeded as Permanent Secretary by Richard Heaton.
