She is portrayed as the daughter of Michael Corleone and Kay Adams and sister of Anthony Vito Corleone.

Mary, although close to her father, is sheltered from the violent world of the Corleone crime family. She falls in love with her cousin, Vincent 'Vinnie' Mancini-Corleone, who is also Michael's hotheaded protegé and heir to the business. Michael disapproves of the romance, believing her marriage to his successor would put Mary in danger. After being appointed Godfather toward the end of the film, Vincent ends the relationship to protect her. She is killed anyway, however, by an assassin trying to kill Michael.

Michael is devastated by Mary's death, which happens as he is beginning to finally put his life as Don Corleone behind him and rekindle his estranged relationship with Adams. A broken man, he retires to Sicily, where he eventually dies.