Johnson is the daughter of former Conservative MEP Stanley Johnson and artist Charlotte Johnson Wahl (née Fawcett), the daughter of Sir James Fawcett, a prominent barrister and president of the European Commission of Human Rights. She is the younger sister of Boris Johnson, the Mayor of London; older sister of Jo Johnson, chief of policy for the Conservative party, MP for Orpington and award-winning editor of the Lex Column on the Financial Times, and older sister of Leo Johnson, the entrepreneur, film-maker and partner at PricewaterhouseCoopers.
On her father's side, Johnson is great-granddaughter of Ali Kemal Bey, a liberal Turkish journalist and the interior minister in the government of Damat Ferid Pasha, Grand Vizier of the Ottoman Empire, who was murdered during the Turkish War of Independence in 1922. During World War I, her grandfather and great aunt were recognised as British subjects and took their grandmother's maiden name of Johnson.
She was educated at Winsford First School on Exmoor, Primrose Hill Primary in Camden, north London, the European School in Brussels, the independent Ashdown House School in East Sussex, Bryanston School in Dorset and St Paul's Girls' School. In 1984 she went to New College, Oxford to read Classics (Literae Humaniores), where she graduated with a 2:1. She edited Isis, the magazine for Oxford University students.
In 1989 she joined the staff of the Financial Times, becoming the first female graduate trainee at the paper, where she wrote on the economy. She moved to the BBC in 1994, but left to move to Washington DC as a columnist and freelancer in 1997. She has written weekly columns for The Sunday Telegraph, The Daily Telegraph, the Evening Standard and other regular columns for Easy Living and She magazines, as well as the Financial Times. She is a contributing editor of The Spectator and until 2009 was a weekly columnist on The Sunday Times and the Evening Standard among other publications. She now writes a weekly column in The Mail on Sunday, and a column for The Big Issue.
In April 2014, she was a judge in the BBC Woman's Hour power list 2014. 
In 2009 Johnson became the ninth editor of The Lady, a weekly magazine established in 1885. Her first few months were the subject of a Channel 4 documentary called The Lady and the Revamp, nominated for a Grierson award. 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 regular contributors such as Mary Killen and Alexander Chancellor, as well as overseeing a redesign by Creative Director Stefano Arata, to better compete with the mainstream women’s magazines. In March 2013 she presented an hour-long documentary for BBC4 called How to Be a Lady: An Elegant History. In March 2014, she will appear in Famous, Rich and Hungry.
Her works include 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 also commissioned and edited The Oxford Myth (Weidenfeld and Nicolson, 1988) while still an undergraduate at Oxford. She is also the author of A Diary of The Lady, My First Year as Editor (Penguin 2010), A Diary of The Lady, My first Year and a Half in 2011, and a new novel, Winter Games was published in 2012. She is a judge of the Women's Prize for Fiction 2013.
- "Family of influence behind Boris Johnson". The Daily Telegraph. 3 May 2008. Retrieved 6 May 2010.
- Spectator article by Norman Stone
- Johnson, Rachel. (6 May 2011). "Rachel Johnson: Boarding school made me". The Telegraph. Retrieved 14 May, 2013.
- "Rachel Johnson", Soho Literary Festival". Retrieved 14 May, 2013.
- Johnson, Rachel. (11 May 2013). "Rachel Johnson: Finally, The Firm has given Camilla the top job she deserves (and I don't care what the bigots say - even if they do scrawl it in green ink)". MailOnline. Retrieved 13 May 2013.
- Noble, Victoria. (20 May 2010). "Interview: Rachel Johnson". The Oxford Student. Retrieved 14 May, 2013.
- "Rachel Johnson". MailOnline. Retrieved 13 May 2013.
- "Rachel Johnson". The Big Issue. Retrieved 13 May 2013.
- "Woman's Hour Power List 2014 – the panel". BBC Radio 4.
- Johnson, Angella (19 July 2009). "Now you can call me The Lady ship: Boris Johnson's sister is the new editor of The Lady magazine". Daily Mail (London).
- "Rachel Johnson to present How To Be A Lady - An Elegant History on BBC Four". (1 February 2013). BBC. Retrieved 13 May 2013.
- Groskop, Viv. (10 October 2010). "A Diary of the Lady: My First Year As Editor by Rachel Johnson". The Observer. Retrieved 13 May 2013.
- Johnson, Rachel. (2011). A Diary of The Lady, My first Year and a Half. Penguin Books. ISBN 9780718192327
- Day, Elizabeth. (24 March 2013). "Winter Games by Rachel Johnson". The Observer. Retrieved 13 May 2013.
- "Rachel Johnson". Women's Prize for Fiction. Retrieved 13 May 2013.
- "John Updike wins special Bad Sex in fiction prize". AP. 2008-11-25. Retrieved 2008-11-26.
- Personal website "Mummy Diaries"
- Authors On Tour Live Podcast Johnson discusses her novel Notting Hell
- How To Be A Lady: An Elegant History with Rachel Johnson