She was educated at Winsford First School, Primrose Hill Primary, the European School in Brussels, the independent Ashdown House School in East Sussex, Bryanston School and St Paul's Girls' School. In 1984 she went up to New College, Oxford to read Classics. She edited Isis, a magazine for Oxford University students .

In 1989 she joined the staff of the Financial Times, becoming the first female graduate trainee at the paper. She moved to the BBC in 1994, and turned freelance in 1996. She has written weekly columns for the Sunday Telegraph, The Daily Telegraph, the Evening Standard and other regular columns for Easy Living magazine, She Magazine, and the Financial Times. She is a contributing editor of The Spectator and contributes a weekly column to The Sunday Times and the Evening Standard and other publications. She regularly appears on radio and TV.

The Lady

In 2009 Johnson became the ninth editor of The Lady, a weekly magazine established in 1885. Her predecessor, 72-year-old Arline Usden left after eighteen years in the job, but has remained with the magazine as editor-at-large. Johnson's arrival was the subject of a Channel 4 fly-on-the-wall documentary The Lady and The Revamp in whch she dismisses her new family-run publication as a “piddling magazine nobody cares about or buys.”

Since taking up the post in September 2009 she has redeveloped the brand, introducing well-known contributors (including Justin Webb and Sir Tim Rice) and a redesign by Creative Director Stefano Arata, to better compete with the mainstream women’s magazines. However, in doing so, Johnson has clashed with The Lady's partial and long-time owner, Julia Budworth, though with only a 20% stake, Budworth has no influence over editorial - or the editor of the magazine - and it is her son, Ben Budworth, who is the managing director.

Under Johnson, a great number of long-standing staff have been sacked and and a major shake-up of every section has bought about the conflict with Budworth. As the latter states, “I put my own money into The Lady to help cover the costs of paying-off people and so on, and Rachel wouldn’t be sitting at her desk if it weren’t for me - and then she turns round and spits in my eye.” Johnson for her part claims that, as circulation is up 7% against falling sales of 15% in the women’s magazine market, she is fulfilling her brief and her aim is to double the readership within eighteen months. As she says of her revamp plans:

"Inside there will be fewer short stories, gardening features and articles about exhibitions. Instead, we’ll concentrate on good reads - more hard-hitting and edgy pieces by well-known writers. We want to attract women aged 45 and up who want more out of their magazines than celebrity pieces and high fashion. What we want to create is an intelligent and articulate women’s magazine with attitude."

Her works include the bestselling Notting Hell (Penguin 2006), a novel about couples living in the Notting Hill area of London, Shire Hell (a follow up to Notting Hell), and The Mummy Diaries (Penguin 2004), a diary of her London-Exmoor year. She edited The Oxford Myth (Weidenfeld and Nicolson, 1988) 'while still an undergraduate at Oxford.

Johnson's Shire Hell won the 2008 Bad Sex in Fiction Prize, which she characterized as an "absolute honour".

Her short story "Severely Gifted" appeared in The Sunday Times on 21 December 2008. ( online text)

She is married to Ivo Dawnay, the communications director of the National Trust, and lives in London with her three children.