Margaret Haig Thomas was the only daughter of David Alfred Thomas and Sybil Haig, was born at Princes Square, Bayswater, on 12th June 1883. She was educated at Notting Hill High School and St Leonards School.
According to her biographer, Deirdre Beddoe: "She received a sound academic education, but there really never was any serious expectation that a girl of her class would work for a living. On leaving school she took the next logical step in the career progression of an upper-class girl and came out. Chaperoned by her long-suffering mother, she endured three successive London seasons. Paralysed by shyness and incapable of small talk, she found this an agonizing experience and she took herself off to Somerville College, Oxford, primarily to escape the horrors of a fourth London season, but gave that up and returned after less than a year."
Margaret married Humphrey Mackworth in 1908. Four months later she joined the Women's Social and Political Union (WSPU). She became secretary of the Newport branch and invited speakers such as Emmeline Pankhurst and Annie Kenney to Wales. During the 1910 General Election she attacked the car of Herbert Asquith. A supporter of the WSPU's arson campaign, she was sent to prison for trying to destroy a post-box with a chemical bomb. However, a hunger-strike led to her early release.
On the outbreak of the First World War, Margaret accepted the decision by the WSPU leadership to abandon its militant campaign for the vote. For the next of couple of years she worked closely with her father, who was sent by David Lloyd George to the United States to arrange the supply of munitions for the British armed forces. In May 1915, Margaret was returning from the United States on the Lusitania when it was torpedoed by a German submarine. Although over a thousand passengers died, Margaret was one of those fortunate enough to be rescued.
Awarded the title Lord Rhondda, David Alfred Thomas was appointed Minister of Food in 1917. Margaret was also given a government post as Director of of Women's Department of the Ministry of National Service. Her report on the Women's Royal Airforce in 1918 led to the dismissal of its commander, Violet Douglas-Pennant and her replacement by Helen Gwynne-Vaughan.
On the death of her father David Alfred Thomas in July 1918. As Deirdre Beddoe points out: "Margaret inherited his property, his commercial interests, and his title. The Directory of Directors for 1919 listed Viscountess Rhondda, as she now was, as the director of thirty-three companies (twenty-eight of them inherited from her father) and chairman or vice-chairman of sixteen of these. Already a famous figure whose activities were widely reported in the London press on account of her business career and of her increasingly leading role as a spokeswoman for feminism, her campaign to take her seat in the House of Lords attracted a great deal more publicity.... But although in 1922 she seemed to have won, when the committee of privileges accepted her plea for admission, the decision was reversed in May 1922."
Lady Rhondda divorced her husband and set up home with Helen Archdale. According to Archdale's biographer, David Doughan: "Helen Archdale had an intense relationship with Lady Rhondda, which seems to have begun in committee work during the First World War, though they also shared a background in suffrage militancy. By the early 1920s, she was sharing an apartment, and, together with her family, a country house (Stonepits, Kent) with Lady Rhondda."
In 1920 Lady Rhondda founded the political magazine Time and Tide. It was initially edited by her lover, Helen Archdale. In 1921 she launched the Six Point Group of Great Britain, which focused on what she regarded as the six key issues for women: The six original specific aims were: (1) Satisfactory legislation on child assault; (2) Satisfactory legislation for the widowed mother; (3) Satisfactory legislation for the unmarried mother and her child; (4) Equal rights of guardianship for married parents; (5) Equal pay for teachers; (6) Equal opportunities for men and women in the civil service.
At first Time and Tide supported left-wing causes but over the years the magazine, like its owner, moved to the right. As David Doughan points out: "However, philosophical disagreements, as well as Lady Rhondda's increasing editorial interventions, resulted in her being effectively forced out of the editorship of Time and Tide in 1926. Although she remained a director of Time and Tide Publishing Company, after her resignation specifically feminist concerns were gradually marginalized in Time and Tide."
Lady Rhondda did not allow politics to get in the way of good writing and contributors to the magazine included D. H. Lawrence, Rebecca West, Vera Brittain, Winifred Holtby, Virginia Woolf, Crystal Eastman, Charlotte Haldane, Storm Jameson, Nancy Astor, Margaret Bondfield, Margery Corbett-Ashby, Charlotte Despard, Emmeline Pankhurst, Eleanor Rathbone, Olive Schreiner, Helena Swanwick, Margaret Winteringham, Ellen Wilkinson, Ethel Smyth, Emma Goldman, George Bernard Shaw, Ernst Toller, Robert Graves and George Orwell. However, it never sold well and it is estimated that during the thirty-eight years she lost over £500,000 on the magazine.
As well as editing Time and Tide, Lady Rhondda wrote a memoir of her father and an autobiography, This Was My World (1933). After breaking up with Helen Archdale she moved in with Theodora Bosanquet, the secretary of the International Federation of University Women.
Margaret Haig Thomas, Lady Rhondda, died in the Westminster Hospital on 20th July 1958.