Freni was born Mirella Fregni into a working class family in Modena (both her mother and tenor Luciano Pavarotti's worked in the same cigarette factory in that city). She was a musically gifted child and sang "Un bel dì vedremo" in a radio competition at age ten. The tenor Beniamino Gigli warned her, however, that she risked ruining her voice and advised her to give up singing until she was older. She resumed singing at age 17.
Mirella made her operatic debut in Modena at 1955, at age 19, as Micaëla in Carmen. She was offered many roles after this, but she decided to put her career aside and marry her singing teacher, Leone Magiera, and have a child.
She resumed her career in 1958 by winning a singing competition and singing Mimì in Turin. She then sang with the Netherlands Opera during the 1959-60 season. Her international breakthrough came when she sang Adina in Franco Zeffirelli's staging of L'Elisir d'Amore at Glyndebourne, where she also sang Susanna and Zerlina during the 1960-62 seasons.
In 1961, Freni made her Royal Opera House debut as Nannetta in Falstaff. In 1963, she made her debut at La Scala, in a production staged by Zeffirelli and conducted by Herbert von Karajan (Freni went on to become one of Karajan's favourite singers, and she collaborated with him in numerous operas and concerts). In 1965, Freni made her Metropolitan Opera debut as Mimi in La Bohème.
In the early 1970s, Freni began singing heavier roles, notably Elisabetta in Don Carlo, Desdemona in Otello and Amelia in Simon Boccanegra.
In 1981, she married Ghiaurov, one of the leading operatic basses of the post-war period.
Freni published a memoire, Mio Caro Teatro in 1990. She was also awarded the order Cavaliere della Gran Croce della Repubblica Italiana that year and the French Légion d'Honneur in March 1993.
Freni continued to add to expand her repertory well into the 1990s, taking on the title roles of Adriana Lecouvreur in Milan, Paris and New York and Giordano's Fedora in Milan. In 1998, she performed the latter's Madama Sans-Gêne in Catania.