After early training in Brescia and possibly some years spent in Mantua, he moved to Rome, where he was employed by Cardinal Cristoforo Madruzzo until 1578, evidently as a singer; after the cardinal's death he served at the court of Cardinal Luigi d'Este, during which time he began to establish a reputation as a composer. By 1581 his music had become immensely popular, as shown by the frequency with which his published books of madrigals were reprinted, and also by the increasingly common appearance of his madrigals in anthologies. In 1587 he moved to Florence where he entered the service of Ferdinando I de' Medici for two years; in 1589 he returned to Rome, where he spent most of his last years, except for a trip to Poland from 1596 to 1597, during which time he was employed at the court of Sigismund III Vasa in Warsaw. According to some sources the trip to Poland ruined his health, and he died in Rome in 1599, shortly after returning from Poland.
Marenzio published at least fifteen collections of music, mostly madrigals but also canzonette and villanelle (related secular a cappella forms very much like madrigals, but usually a bit lighter in character). Close to 500 separate compositions survive. Stylistically, his compositions show a generally increasing seriousness of tone throughout his life, but in all periods he was capable of the most astonishing mood-shifts within a single composition, sometimes within a single phrase; rarely does the music seem disunified, since he closely follows the texts of the poems being sung. During his last decade he not only wrote more serious, even sombre music, but experimented with chromaticism in a daring manner surpassed only by Gesualdo. In one madrigal (O voi che sospirate a miglior note) he modulated completely around the circle of fifths within a single phrase, using enharmonic spellings within single chords (for instance, simultaneous C sharp and D flat), impossible to sing unless some approximation of equal temperament is being observed.
Even more characteristic of his style, and a defining characteristic of the madrigal as a genre, is his use of word-painting: the technique of mirroring in the music a specific word, phrase, implication or pun on what is being sung. An obvious example would be a setting of the phrase "sinking in the sea" to a descending series of notes, or accompanying the word "anguish" with a dissonant chord followed by an unsatisfying resolution.