Gwendolen was born in 1881. She was the daughter of William Hillier Onslow, 4th Earl of Onslow (1853–1911), and Florence Coulston Onslow, née Gardner (1853–1934).
Census 31 March 1901: Living with her parents and three siblings in West Clandon, Surrey, England. Aged 19. No occupation recorded.
Gwendolen was a Conservative politician in the United Kingdom, and, by marriage, a member of the Anglo-Irish Guinness brewing dynasty.
She married Rupert Edward Cecil Lee Guinness, 2nd Earl of Iveagh (son of Edward Cecil Guinness, 1st Earl of Iveagh and Adelaide Maria Guinness) on 8 October 1903 at St. George Hanover Square, London, England.
She was appointed Commander, Order of the British Empire (C.B.E.) in 1920.
Her husband was a Conservative Member of Parliament (MP) for Southend. In 1927, he ceased to be an MP when he succeeded to his father's earldom.
The Countess of Iveagh, as Gwendolen Guinness was now known, won the Southend by-election on 19 November 1927 to replace her husband as MP. She served until her retirement at the 1935 general election. When she retired she was succeeded as MP by her eldest daughter Honor Guinness's husband, Henry "Chips" Channon.
In 1956 she presented her Surrey childhood home, Clandon Park, to the National Trust.
She passed away in 1966.
St Andrew and St Patrick Churchyard, Elveden, Forest Heath District, Suffolk, England
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