Maria made her first professional appearance in von Suppe’s Boccacio in 1939 and sang the leading part in Tosca in 1942. After a series of disappointments and resenting envy by her colleagues, she returned to the American land of her birth. She lived there for two years (1945-1947), met the bass Nicola Rossi – Lemeni and through him came to know the tenor Giovanni Zenatello, artistic director of the Arena of Verona. Maria Kalogeropoulos left for Italy, met the industrialist Giovanni Batista Meneghini, married him, became Maria Callas and experienced her artistic life’s curtain raiser without suspecting that she was about to emerge as the lyric theatre’s most brilliant personality.
FROM http://www.hri.org/MFA/thesis/autumn97/callas.html

Callas’ ability to sing such a wide range of roles was one of the things that led to her meteoric rise. But critic Conrad Osborne says it also contained the seeds of her vocal decline. Callas’ voice was already starting to fail her by the time she was in her 40s — quite young for an opera singer. A number of factors, including a rapid loss of weight, may explain why…But for Callas fans like James Jorden, the diva more than made up for her vocal flaws with her talent for bringing the music to life.
FROM http://www.npr.org/templates/story/story.php?storyId=123612228

By the time of her retirement, she had performed more than 40 different roles and had recorded more than 20 complete operas. Callas’s personality and philosophy of performance are powerfully depicted in Terrence McNally’s play Master Class (first performed and published 1995), based on her classes at Juilliard.
FROM http://www.britannica.com/biography/Maria-Callas