Born Mary Theresa Olivia Cornwallis-West at Ruthin Castle in Denbighshire, Wales, she was the daughter of Col. William Cornwallis-West (1835–1917) and his wife, Mary "Patsy" FitzPatrick (1856–1920). Her father, born William West, was a great-grandson of John West, 2nd Earl De La Warr. Her mother was a daughter of Reverend Frederick FitzPatrick and Lady Olivia Taylour, herself daughter of the 2nd Marquess of Headfort.
During her marriage, Daisy, known in German as the Fürstin von Pless, became a social reformer and militated for peace with her friends William II, German Emperor and King Edward VII of the United Kingdom. During World War I she served as a nurse.
After her divorce at Berlin on 12 December 1922 she published a series of memoirs that were widely read in the United Kingdom, the United States, and, in the German language, on Continental Europe.
Hans Heinrich married as his second wife, at London on 25 January 1925, Clotilde de Silva y González de Candamo (1898–1978). This marriage produced two children, and was annulled in 1934. Subsequently Clotilde married her stepson, Bolko, and was the mother of Daisy's and Hans Heinrich's only grandchildren.
Daisy's brother George in 1900 married Jennie Churchill, the mother of Winston Churchill, as his first wife, and after their divorce married in 1914 Mrs. Patrick Campbell, the actress, as his second. Her sister, Constance, married in 1901 Hugh Grosvenor, 2nd Duke of Westminster, and after their divorce she married in 1920 James FitzPatrick Lewes.
On 8 December 1891, in London, she married Hans Heinrich XV, Prince of Pless, Count of Hochberg, Baron of Fürstenstein (1861–1938), one of the wealthiest heirs in the German Empire, becoming châtelaine of Fürstenstein Castle and Pless Castle in Silesia.
The couple had four children:Daughter (25 February 1893 – 11 March 1893).
Hans Heinrich XVII William Albert Edward (2 February 1900 – 26 January 1984), Prince of Pless, Count von Hochberg and Baron of Fürstenstein. Married twice but had no issue.
Alexander Frederick William George Conrad Ernest Maximilian (1 February 1905 – 22 February 1984), Prince of Pless, Count von Hochberg and Baron of Fürstenstein. Unmarried and childless.
Bolko Conrad Frederick (23 September 1910 – 22 June 1936), who later caused an scandal by marrying his stepmother Clotilde de Silva y Gonzáles de Candamo (Hans Heinrich XV's second wife).
The Princess maintained her links with English society, appearing with her children in Country Life magazine.
The Princess of Pless was a Dame of the Order of Theresa of Bavaria and of the Order of Isabella the Catholic of Spain, and was awarded the German Red Cross Decoration.
Daisy, Princess of Pless, died in 1943 in relative poverty at Waldenburg, today Wałbrzych, Poland.