Born in Mons, his first known position was with Ferrante Gonzaga of Mantua, whom he served from around 1544. He then worked as a singer and a composer for Constantino Castrioto in Naples and for the Archduke of Florence. In 1553, he became chorus leader of St. John Lateran in Rome, but he stayed there only for a year.
He then returned to the Low Countries and had his works published in Antwerp (1555-1556). In 1556 he joined the court of duke Albrecht V of Bavaria. He was appointed maestro di capella in 1563 and served Albrecht V and his heir, Wilhelm V, until his death.
One of the most prolific and versatile composers of his time, Lassus wrote over 2000 works in all Latin, French, Italian and German vocal genres known in his time. These include 530 motets, 175 Italian madrigalss and villanellas, 150 French chansons, and 90 German lieder.
Almost 60 masses have survived complete; most of them are parody masses based on secular works written by himself or other composers. Among his other liturgical compositions are hymns, canticles (including over 100 Magnificats), responsories for Holy Week, Passions, Lamentations, and some independent pieces for major feasts.
Lassus died in Munich.