In April 1944 Resnik won the Metropolitan Opera Auditions of the Air, performing "Ernani, involami", and was offered a contract with that company for the 1944/45 season. Her debut at the Met was doubly dramatic – on one day's notice she substituted on December 6, 1944, for Zinka Milanov as Leonora in "Il trovatore" eliciting acclaim from the public, the critics noting that all the vocal "virtuosity" and her stage presence as an actress were very impressive. During the next decade, she offered twenty heroines: Donna Elvira and Donna Anna ("Don Giovanni"), "Fidelio", Sieglinde ("Die Walküre"), Gutrune ("Götterdämmerung"), Chrysothemis ("Elektra"), Rosalinda and Eboli ("Don Carlos"), "Aida", Alice Ford ("Falstaff"), "Tosca", "Madama Butterfly" and Musetta ("La bohème"). She was the Met's first Ellen Orford in "Peter Grimes" and created Delilah in the world premiere of Bernard Rogers' "The Warrior". She then also began a long association with the San Francisco Opera. As for the voice, it was a dramatic soprano which invited comparison with Rosa Ponselle. During these years, her teacher was Rosalie Miller and her life began with the legendary conductors Otto Klemperer, Bruno Walter, George Szell, Fritz Reiner, William Steinberg and Erich Leinsdorf.