Resnik made her professional singing debut at the age of 22 on October 27, 1942 giving a recital of art songs at the Brooklyn Academy of Music. Just two months later she made her professional opera debut with Fritz Busch's New Opera Company at the Broadway Theatre in Manhattan as Lady Macbeth in Giuseppe Verdi's "Macbeth" with Jess Walters in the title role. In February and March 1943 she sang Leonore in "Fidelio" and Micaela in "Carmen" at the Palacio de Bellas Artes in Mexico City under Erich Kleiber. In the Spring of 1944 she portrayed both Frasquita and Micaela in Georges Bizet's "Carmen" in the New York City Opera's (NYCO) first season with Dusolina Giannini in the title role. She was also seen at the NYCO that season as Santuzza in Mascagni's "Cavalleria rusticana".