Margaret Haig Thomas was the only daughter of David Alfred Thomas and Sybil Haig, was. She was educated at Notting Hill High School, St. Andrews and Somerville College, Oxford.
Margaret married Humphrey Mackworth in 1908. Four months later she joined the Women's Social and Political Union (WSPU). 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, Margaret attempted to take her seat in the House of Lords as Viscountess Rhondda, but was kept out after extensive legal proceedings.
In 1920 Lady Rhondda founded the political magazine Time and Tide. Edited initially by Helen Archdale, Rhondda took over in 1926. At first Time and Tide supported left-wing causes but over the years the magazine, like its owner, moved to the right. Lady Rhondda did not allow politics to get in the way of good writing and found famous contributors to the magazine. 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).