The cabinet secretary is the prime minister’s most senior policy adviser and sits next to them during cabinet minister meetings, which usually take place weekly in Downing Street when Parliament is sitting.
They are responsible for ensuring policy decisions are put into action and is ultimately responsible for all civil servants in Great Britain.
Dame Antonia has a long track record of leading government departments including spells as permanent secretary at the Home Office, International Trade and the Ministry of Justice.
Her appointment was made following a due diligence process overseen by crossbench peer Baroness Gisela Stuart, the First Civil Service Commissioner.
Sir Keir said Dame Antonia is an “outstanding public servant, with a 25‑year record of delivering for the British people”.
He said: “Since becoming prime minister, I’ve been impressed by her professionalism and determination to get things done.”
Dame Antonia said it was a “huge privilege” to be appointed, adding: “The Civil Service is a great and remarkable institution, which I love.
“We should be known for delivery, efficiency and innovation, working to implement the government’s agenda and meet the challenges the country faces.”
Dame Antonia, a University of Oxford graduate, joined the Civil Service in 2000 as an economist after previously working for a consultancy firm in the private sector.
Her high-ranking jobs included serving as the UK’s consul general in New York between 2016 and 2017.
Dame Antonia’s job involved promoting UK trade and business in New York, which saw her pictured with celebrities, shortly after the referendum to take Britain out of the EU.
Concerns were raised about Dame Antonia’s use of social media, with one former colleague claiming in an email that she seemed to be “focusing an undue amount on building her personal Twitter brand”.