Cook made her professional opera debut on October 31, 1965, at the Academy of Music with the Philadelphia Lyric Opera Company singing the title role in a concert version of Donizetti's Lucia di Lammermoor with Frank Guarrera as Enrico. However, this performance did not excite the attention that Cook was hoping for and she was forced to take work outside of music for the next several years while continuing with further studies with Williams, and Nicola Palumbo in New York City. She appeared in many concerts and performances in the Philadelphia area during the late 1960s including portraying Galatea in Handel's Acis and Galatea with John Darrenkamp as Damon at the Philadelphia Museum of Art in 1967 and returning there in 1968 to sing Vespina in Haydn's L'infedeltà delusa, both with the Concerto Soloists Chamber Orchestra of Philadelphia.
Cook made her professional opera debut on October 31, 1965, at the Academy of Music with the Philadelphia Lyric Opera Company. She sang as Lucia di Lammermoor and was accompanied by Frank Guarrera as Enrico. This performance did not achieve Cook's desired attention, so she took work outside of music for several years while continuing her studies with Williams and Nicola Palumbo in New York City. During the late 1960s, Cook performed in concerts and events in Philadelphia, including Galatea in Handel's Acis and Galatea with John Darrenkamp as Damon at the Philadelphia Museum of Art in 1967 and Vespina in Haydn's L'infedeltà delusa with the Concerto Soloists Chamber Orchestra of Philadelphia in 1968.