[4] Gramont excelled in mathematics and graduated first from the École spéciale militaire de Saint-Cyr.

After attending the École, he entered the French cavalry and became a Second lieutenant in the 4th Hussars, a regiment in which he served during the Franco-Prussian War of 1870, during which time his father was France's Minister for Foreign Affairs.

He was also known as the Prince of Bidache (the principality of Bidache maintained de jure sovereignty from 1570 until 1790 when, by royal edict, the territory of the principality was declared to be a part of France by Louis XVI, although his ancestor, Antoine de Gramont, wasn't ousted until 1793).

[9] After becoming Duke, he resided at the Château de Mangé, in Verneil-le-Chétif in Sarthe, bequeathed by his father-in-law, the Prince of Beauvau, to his granddaughter, Élisabeth.

[10] Upon marrying his second wife, who had inherited substantial property after the death of her father in 1886, the newly wealthy Duke decided to restore his family's former seat, the Château de Bidach.

His first marriage, when he was styled Duke of Guiche, was on 20 April 1874 in Paris to Princess Isabelle Marie Blanche Charlotte Victurnienne de Beauvau-Craon (1852–1875), who reportedly gave up marrying the very rich Count de Gramont d'Aster to marry Agénor.

The Gramont family was considered de László's "greatest friends and patrons in Europe".