Although de Harlay belonged to the Protestant branch of the family of Harlay, he adopted the Catholic religion in 1572 during the massacres of the Huguenots. In the second half of the 16th century, de Harley served as the French ambassador at the court of Sultan Selim II in Constantinople, where he acquired a number of diamonds that had been mined near Golconda, now in the Indian state of Telangana. Among these jewels were some of the largest diamonds ever seen in Europe, such as the Sancy and its smaller sibling the Beau Sancy. In 1589 he obtained in Geneva and Berne sums sufficient to raise an army of mercenaries for Henry III, partly by the sale of a number of his jewels. Henry IV of France made him superintendent of his finances in 1594, but in 1599 he was replaced by Maximilien de Béthune, duc de Sully. Meanwhile he had been a second time converted to Catholicism, but his influence at court waned, and he retired from public life in 1605. He survived until 13 October 1629, leaving a "Discours sur l'occurrence des affaires".