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 replaces Sir Chris Wormald, who was forced out as cabinet secretary last week after serving in the job for just 14 months.
Sir Chris is the shortest serving cabinet secretary in history.
His departure came after months of negative media reports suggesting Downing Street was unhappy with his performance.
At the time of his appointment, the prime minister said Sir Chris would be tasked with “the complete re-wiring of the British state to deliver bold and ambitious long-term reform”.
As a career civil servant, some questioned whether he was the best person to reform the Civil Service.
Sir Chris also had ultimate responsibility for the due diligence checks carried out before Lord Mandelson’s appointment as US ambassador, although he took up the role only a few days before the appointment was formally announced.
Lord Mandelson was sacked from the role last September afrer Downing Street said new information about the depth of his relationship with paedophile financier Jeffrey Epstein had emerged.