Scholars know little of Adriaen Thomasz. Key's background, but he was probably portraitist Willem Key's pupil. Key was either distantly related to Willem or simply took his teacher's name.
Key became a master in Antwerp's Guild of Saint Luke in 1568, and his dated paintings are known from 1572 onwards. After Catholic Spain completed its reconquest of the Netherlands by seizing Antwerp in 1585, Key remained registered as a Calvinist yet continued working and teaching pupils. There is no record of him after 1589. Key is most admired for his portraits and for his power of objective observation. In his lifetime, he must have been highly respected, for he painted numerous portraits of William the Silent, prince of Orange and count of Nassau, a great leader in the Netherlands's struggle for independence from Spanish rule. His later portraits, which primarily depict Antwerp citizens, display greater emphasis on the sitters' status, probably resulting from Antonis Mor's influence. Key also painted altarpieces for Antwerp churches, including the high altar of the Franciscan church, whose figures resemble those of Willem Key. His chiaroscuro woodcuts show awareness of the work of Flemish Italianate artist Frans Floris.