Mary Borden, the daughter of the wealthy businessman, William Borden, was born in Chicago on 15th May 1886. After attending Vassar College (1904–7) went on a chaperoned world tour, where she met a Scottish missionary called George Douglas Turner. They were married in Lausanne, Switzerland, in 1908. A daughter was born in Maine in 1909 and a second in Kashmir in 1910.
While living in London had an affair with Percy Wyndham Lewis. She also joined the Women Social & Political Union. Using the pseudonym Bridget Maclagan she published two novels, The Mistress of Kingdoms (1912) and Collision (1913). One critic described the books as autobiographical with feminist undertones.
On the outbreak of the First World War in 1914, Borden she set up a hospital unit in Dunkirk. She remained in France, running at her own expense a mobile hospital on the Western Front. While at the Somme she met and fell in love with Edward Louis Spears, a French liaison officer. Borden, who was awarded the Croix de Guerre by the French government, remained with the unit until 1918.
After she obtained a divorce they were married on 31st March 1918 at the British embassy in Paris. According to her biographer, Nicola Beauman: "There was much gossip that Spears was only marrying her for her money (which was in part true) and that he had broken up a happy home (also partly true)."
Edward Louis Spears retired from the British Army in 1920 and with the encouragement of Winston Churchill in November, 1922, stood as the National Liberal candidate for Loughborough. Elected to the House of Commons he served until defeated in October, 1924.
In 1928 Borden published the novel Flamingo. According to one critic, the novel "is a love story which should live as long as any woman believes in love". She followed this with The Forbidden Zone, an account of her experiences during the First World War. A novel about a nurse on the Western Front, Sarah Gay, appeared in 1931.
Edward Louis Spears followed Winston Churchill into the Conservative Party and was elected for Carlisle in the 1931 General Election. The couple moved to St Michael's Grange, Warfield, Berkshire. They also had a home at 12 Strathearn Place, Bayswater. The marriage was not a success. According to Nicola Beauman "her life became increasingly blighted by her domineering husband's blatant devotion to his mistress" and he "was abusive and cruel". Max Egremont argues that his mistress was Nancy Maurice (1901–1975), daughter of Major-General Sir Frederick Barton Maurice, who worked as Spears's secretary.
Borden continued working as a novelist and wrote the controversial The Techniques of Marriage (1933), Mary of Nazareth (1933), The King of the Jews (1935), and a collection of short-stories, Passport for a Girl (1939), an account of English attitudes to the rise of Adolf Hitler in Nazi Germany.
During the Second World War ran a mobile field hospital in the Middle East and wrote about this in Journey Down a Blind Alley (1946). For the Record (1950) is about a secret agent and Martin Merriedew (1952) about a pacifist tried for treason. In the 1950s May often travelled to the United States, partly in order to visit her nephew Adlai Stevenson.
Mary Borden died of heart failure on 2nd December 1968 at her home, St Michael's Grange, Warfield, Berkshire. The following year Edward Louis Spears married his long-term mistress, Nancy Maurice. According to Max Egremont: "Mary Borden had a life of her own as a writer whereas Spears was Nancy Maurice's whole existence and she worked ruthlessly to further his career, encouraging him in the pursuit of vendettas and strong dislikes."