He was born in Valenciennes, where he probably received his early musical training. Sometime fairly early in life he became a Protestant. The first record of his musical activity is from 1552, when four chansons attributed to him were published at Leuven, in anthologies of works by several composers. In 1564 he moved to Paris, where he became acquainted with the Huguenots. By this time he had already acquired some international fame, as evidenced by the appearance of his name in a list of "contemporary composers of excellence" in a manuscript copy of the "Penitential Psalms" of Orlande de Lassus, which were probably composed in the 1560s in Munich. Lassus may have met Le Jeune in the mid-1550s during a trip to France; however this has not been definitely established.