Carrie Symonds has become Downing Street's "First Girlfriend".
The ex-Tory spin doctor will be Number 10's first live-in girlfriend since Martine McCutcheon's appearance as the prime minister's PA in Love Actually.
Dressed in a £120 daisy, red dress from Ghost - which has now sold out in all sizes except XXS - Miss Symonds smiled as she watched Mr Johnson deliver his first speech as prime minister outside Number 10.
Miss Symonds was nestled among a crowd of advisers and staff - but the 31-year-old could now find herself increasingly in the spotlight.
Here's what some of her predecessors had to say about life at Number 10:
The mother-of-four was a successful businesswoman and creative director for Bond Street stationers Smythson until her husband became PM in May 2010 - and she became a part-time consultant.
She told the Mail on Sunday in 2015: "The truth is I am not a very hands-on political wife; I don't get involved in day-to-day Downing Street life.
"They don't need me interfering, but in the evening we will talk about each other's day.
"I try to stay out of the Westminster village. There are times when I will be surprised and curious about what's been announced.
"I hope that he can get an everyday, down-to-earth view from me, as a mother who works in business."
A campaigner for global health and education, the mother-of-three published a memoir in 2011 entitled Behind the Black Door.
She wrote that former Italian prime minister Silvio Berlusconi once asked her for supermodel Naomi Campbell's phone number, and that the only time she was ever star-struck was when she met Barbra Streisand.
In an interview with the Daily Mail in 2011, she said: "When I look back on it, maybe the one thing I would have changed about my role is that I would have spoken up more.
"When things were going wrong, and I was reading stories about him that simply were not true, I wonder whether I should have got more involved - to say: 'this is just not the case'.
"The one thing that I could have done to make a difference was to be more vocal."
A barrister, lecturer and writer, Mrs Blair was famously thrust into the media glare the morning after Labour's 1997 election victory when she was photographed opening the door in her bed clothes.
Reflecting on that moment, she told The Guardian in May 2015: "Do I have any advice for someone new to No 10? Never open the door in your nightie."
She also revealed she fell pregnant with her son Leo in September 1999 at the Queen's Scottish residence, Balmoral, because she was too embarrassed to pack contraception.
In her 2008 memoir, Speaking For Myself, she wrote: "As usual up there, it had been bitterly cold, and what with one thing and another..."
She said she initially refused to believe she was pregnant and assumed she was going through the menopause.
When her pregnancy was confirmed, she said her husband's reaction was: "Oh my God. We'll have to tell Alistair [Campbell, his communications chief]."