He previously served as Sunak's de facto chief of staff as head of the Joint Economic Unit during his chancellorship.

[1] In February 2020, as head of the Joint Economic Unit between 10 Downing Street and 11 Downing Street, Booth-Smith became the de facto chief of staff to Sunak, who was then Chancellor of the Exchequer,[5] overseeing the Treasury's response to the COVID-19 pandemic in the United Kingdom.

Booth-Smith gained the nickname "Travolta of the Treasury" for wearing a leather jacket with his shirt buttons undone.

[5] Booth-Smith was appointed as Downing Street Chief of Staff in October 2022 after Sunak became prime minister.

[1] In 2023, the New Statesman named Booth-Smith as the ninth-most influential figure in British right-wing politics.