Horne made her debut at the Royal Opera House in October 1964 as Marie in "Wozzeck". Her La Scala debut was as Jocasta in Stravinsky's opéra-oratorio "Œdipus rex" on March 13, 1969. Another of Horne's breakthroughs occurred that same year during a performance of Rossini's "Le siège de Corinthe" at La Scala, when Horne received a remarkable mid-act seven-minute ovation. Horne made her debut at the Metropolitan Opera in 1970 as Adalgisa in Bellini's "Norma" with Sutherland in the title role. She thereafter appeared regularly at the Met, opening the 1972/1973 season as Carmen. A great success there was in Meyerbeer's "Le prophète", in John Dexter's production. In 1984, she sang the title role of Handel's opera seria "Rinaldo" (directed by Frank Corsaro), the first Handel opera ever performed at the Met.