Cat started her career at the accountancy firm PwC, joining the Civil Service in 2013 first at the Legal Aid Agency moving on to become the Group Finance Director at the Ministry of Justice overseeing all aspects of financial management and supporting prison reform, courts reform and transformation programmes. In 2017, Cat joined the Board of the Ministry of Defence as Director General Finance. In this role she was the primary financial advisor to ministers, the Defence Board and Accounting Officer and led the finance, commercial and analytical functions across Defence. During this time, Cat led the reform of Defence Acquisition and Procurement as part of the Modernising Defence Programme and instigated the Financial Improvement Programme. Cat led the oversight of the Defence portfolio, and was the Race Champion for the Department.

Cat joined HM Treasury in 2020 as Director General Public Spending. She took on the role of Acting Permanent Secretary in September 2022, and in October 2022 was appointed to Second Permanent Secretary. In this role Cat oversaw all government public spending, as well as international and national security policy for the department. She combined this with positions as head of the Government Finance Function and Chair of the Finance Leadership Group for Government.

Cat was appointed Chief Operating Officer for the Civil Service and Permanent Secretary to the Cabinet Office in 2024. As well as her departmental role leading the Cabinet Office as its Permanent Secretary, she is responsible for ensuring the high performance and efficiency of the Government system, working closely with other Permanent Secretaries to modernise and improve the Civil Service. She chairs the Civil Service Board and the Civil Service Operations Board.

Outside of work Cat lives in the Kent North Downs, enjoys running, cycling and walking the dog with her wife Ruth.