Those who have worked with her say she had a transformational effect on Buckingham Palace, persuading the Queen to embrace Twitter and Facebook, and modernising her department using “a natural, easy manner, backed with brains and ideas”. She will need to have a similarly transformational influence on Downing Street if Johnson is to fight another election – though if successful, she is unlikely to seek credit, as she despises publicity.

She cringes at the nickname of “Samantha the Panther”, which was invented by the media, purely because it rhymed, during her time at the Palace.

One former colleague said: “I was aware of the name, but I don’t remember anyone ever using it. It does sort of make sense, though, because she has feline grace but will bite hard if you’re out of line.”

The same source said: “That’s not really how she works, though. You will know if something you have done is below par, but the worst thing will be the feeling that you have let her down, not what she says about it.”

When her boss and mentor Sir Christopher (now Lord) Geidt, then the Queen’s private secretary, was ousted in a coup said to have been backed by the Prince of Wales, Cohen resigned in solidarity. But she was so highly regarded by the Queen that Her Majesty personally asked her to stay on until the following Commonwealth summit, then asked her to move to Kensington Palace to solve the conundrum of Meghan Markle.

Cohen stayed for less than 18 months, and was said to have told friends that dealing with the Sussexes was like trying to work with “teenagers”. The idea that the Sussexes would not listen even to Cohen was seen by royal insiders as the ultimate indictment of the couple.

Her name was later dragged into the High Court action in which the Duchess successfully sued Associated Newspapers for publishing extracts of a private letter she had written to her father, Thomas Markle. Cohen was one of the “Palace Four” members of the Sussex’s staff who offered to provide evidence that would “shed some light” on the drafting of the letter.

Lord Geidt, who now works for Johnson as his independent adviser on ministers’ interests, is understood to have been hugely supportive of Cohen. Her new job will see her work alongside Cabinet Secretary Simon Case, known to her from his previous job as Prince William’s private secretary.

Cohen, who is expected to start her job in April, appears to have taken on the role without a precise job description: Downing Street insiders admit the exact parameters of her job are still being worked out, with one source suggesting she will be a “de facto deputy” to Steve “three-jobs” Barclay, the Downing Street chief of staff, Chancellor of the Duchy of Lancaster and MP for North East Cambridgeshire. There is no official title yet, though she is likely to be head of the Prime Minister’s office or director of Government relations.

The mother-of-three lives in west London with her South African husband Richard, who works in the City, and has always put her family first, friends say, working furiously hard during office hours so that she can spend her evenings with her husband and children.

She is currently the chief executive of the Commonwealth Enterprise and Investment Council, as well as being the co-founder of the Queen’s Green Canopy project, and the current co-chairman of Cool Earth, a non-profit organisation working with indigenous communities to stop deforestation.