Guy Louis Jules de Lasteyrie du Saillant was born on 3 October 1879 and from birth until his father's death in 1923, he was referred to as "Count" Guy de Lasteyrie as it was customary to refer to the eldest son of a marquis as "Count.

[1] His family had a home in Paris and in the French countryside, known as Château de la Grange-Bléneau.

His maternal grandparents were Thomas Mills Goodlake, the High Sheriff of Berkshire, and Emilia Maria (née Baker) Goodlake (a daughter of Lt. Col. Sir Edward Baker, 1st Baronet and Lady Elizabeth FitzGerald).[5][a].

During World War I, his wife volunteered with the Red Cross of France by driving an ambulance while he fought with his cavalry regiment at the Front.

[2] On December 19, 1912, Guy was married to sculptor Constance Whitney Warren at St Patrick's Cathedral in New York City.