She was born in Wimbledon into a noted family in society and lived in Cawnpore, India, until the age of 20 when she went on the Grand Tour. In 1901, she married patent lawyer Sir Arthur Colefax, who was briefly the MP for Manchester South West in 1910. They set up home at Argyll House, King's Road, Chelsea and at Old Buckhurst in Kent. Widely admired for her taste, after she had lost most of her fortune in the Wall Street Crash she began to decorate professionally, using her formidable address book for contacts. She was able to purchase the decorating division of the antique dealers Stair and Andrew of Bruton Street, Mayfair and established Sibyl Colefax Ltd in partnership with Peggy Ward, the Countess Munster. On her 'retirement' (following a family tragedy) Peggy Ward advised her to take on John Fowler (1906–1977) as her partner, which she did in April 1938. The advent of war cut short this partnership. During the Second World War, she organised a soup kitchen and continued to entertain. She often held small lunch parties at The Dorchester known as 'Ordinaries' after which the guest would receive a small bill.
In 1944, the business, managed by John Fowler, took a lease on 39 Brook Street, Mayfair where it remained until December 2016. Also in 1944 Sibyl Colefax sold the business to Nancy Tree (Nancy Lancaster as she became in 1948) for a sum in the order of £10000. She renamed the business Sibyl Colefax and John Fowler Ltd, the name continuing today as the decorating division of the Colefax Group Plc.
Colefax died at her home in Lord North Street, Westminster on 22 September 1950. Harold Nicolson penned an affectionate tribute that appeared shortly after in The Listener. She was the inspiration, according to the art historian John Richardson, for the designer Mrs. Beaver in Evelyn Waugh's A Handful of Dust, and for Mrs. Aldwinkle in Aldous Huxley's Those Barren Leaves.
- "Colefax and Fowler: the best in English interior decoration” Jones,C: London, Barrie & Jenkins 1989 ISBN 0-7126-2021-4
- Influence in 1920s
- Her uncle was the eminent constitutionalist Walter Bagehot – The Spectator, p2713, October 1950
- The Times, Tuesday, 26 September 1950; pg. 6; Issue 51806; col E Lady Colefax A Great London Hostess RAB
- "Lady Colefax Knowledge Of Art Princess Marthe Bibesco". The Times (51808, col D). 28 September 1950. p. 6.
- House & Garden, September 1983
- A Passion for Friendship by Kirsty McLeod. Michael Joseph, London, 1991.
- Great Hostesses by Brian Masters. Constable, London, 1982.
- 'John Fowler, Prince of Decorators' by Martin Wood. Frances Lincoln, London 2007.