He succeeded in Armagnac at the death of his brother, John III, in 1391.
After prolonged fighting, he also became Count of Comminges in 1412.
When his brother, who claimed the Kingdom of Majorca, invaded northern Catalonia late in 1389 in an attempt to seize the kingdom's continental possessions (the County of Roussillon), Bernard commanded part of his forces.
Bernard's wife was Bonne, the daughter of John, Duke of Berry, and widow of Count Amadeus VII, Count of Savoy.
Bernard d'Armagnac became the nominal head of the faction which opposed John the Fearless in the Armagnac–Burgundian Civil War, and the faction came to be called the "Armagnacs" as a consequence.
He became constable of France in 1415 and was the head of the government of the Dauphin, the future Charles VII, until the Burgundians invaded Paris in the night of 28-29 May 1418.
On 12 June 1418, he was one of the first victims of the massacres in which over 550 of his real or suspected followers were killed in the course of weeks throughout the summer.