[1] She also had the opportunity to work with Elisabeth Schwarzkopf who took her to Herbert von Karajan, and with mentors Renata Tebaldi, Zinka Milanov and Licia Albanese.
On December 3, 1984, Millo made her Metropolitan Opera debut replacing an ailing soprano as Amelia in Verdi's Simon Boccanegra with James Levine on the podium.
The critics praised Millo's performance with The New York Times proclaiming that her voice had "a breadth and a shining ring that would have won her a midscene ovation in any Italian opera house".
[3] In 1986 Millo made her Carnegie Hall debut with Verdi's I Lombardi alla prima crociata with tenor Carlo Bergonzi and Eve Queler and the Opera Orchestra of New York.
A car accident in Torino briefly sidelined Millo and forced her to cancel Caterina Cornaro in New York; she returned to the stage of the Metropolitan Opera in 1995 and 1996, playing Amelia (Simon Boccanegra) and Desdemona (Otello), opposite Plácido Domingo.
In October 1993 Millo made a debut in the role of Imogene of Il pirata by Vincenzo Bellini at ABAO Bilbao, along Bruno Praticó as Ernesto and Ramón Vargas in the part of Gualtiero.
[8][9] In 1997, Millo sang her first Tosca at the Liceu, Barcelona, with Giacomo Aragall and Juan Pons, followed by performances of the opera at La Scala and at the Met.
[citation needed] Millo returned to the stage in her debut of Puccini's Il tabarro as Giorgetta in Genoa at the Teatro Carlo Felice in 2014.