Costeley was born in
Fontanges-en-Auvergne, coincidentally the same town as contemporary composer
Antoine de Bertrand. Nothing is known of him prior to his arrival in Paris in or before 1554, at which time he met, and became acquainted with the music of, such diverse figures as
Jean Maillard,
Jacques Arcadelt, and
Sandrin. It was through Sandrin, who had recently worked in Italy, that Costeley probably became interested in the latest trends in Italian scholarship, particularly the theories of
Nicola Vicentino, some of which involved composition using
microtones. Costeley's only microtonal composition,
Seigneur Dieu ta pitié, was apparently written at exactly the time that Sandrin was in Paris. During the late 1550s Costeley rose in prominence in Parisian musical life, being published by
Le Roy and
Ballard in 1559. Since Le Roy was closely connected to the royal court through the family of
Catherine de Clermont, who was to become the
Countess of Retz, it is probable that his influence was significant in Costeley's rise. By 1560 Costeley had been appointed to the royal court, as organist, music teacher to the ten-year-old monarch, and composer of chansons for the royal chamber. In 1570 he published
Musique de Guillaume Costeley, which contains almost all of his surviving works. In November of this same year King
Henry III granted a charter for the formation of the
Académie de poésie et de musique, of which Costeley was a founding member; there is, however, no evidence of any musical composition by Costeley between 1570 and his death in 1606. He was lauded by the group and took part in its activities (the king himself was probably a member, and attended some of their meetings, as did his successor
Charles IX after 1574).
Baïf himself, the founder of the Académie, wrote several poems in Costeley's honor. However Costeley was no longer resident full-time at Paris. He had purchased a house in
Évreux in
Normandy, and married; the King only required him to be at court for the first three months of the year. Records of his property purchases indicate that he had become wealthy in service of the king. In 1581 he was made tax assessor at Évreux, and in 1592 his wife died and he married again. In 1597 he was named as an advisor to the king ("Conseiller du Roy"), and he seems to have remained in Évreux in semi-retirement until his death. ==Music and influence==