Born in 1922, she is the long-term girlfriend and eventual wife of Michael Corleone (Al Pacino), the son of Don Vito Corleone (Marlon Brando), and the future Godfather. As a non-Italian, she is somewhat of an outsider from the beginning and symbolizes Michael's initial desire to live a more Americanized life, in contrast to the Corleone family's criminal enterprises.

After Michael kills two men who tried to assassinate his father in 1946, he and Kay are separated while he takes refuge in Sicily, where he briefly marries a local virgin, Apollonia (Simonetta Stefanelli). Apollonia is accidentally killed by a car bomb intended for Michael. He returns to the United States a year later, when he and Kay reunite and marry. Michael becomes the new Don in 1955 after his father's death. Due to Michael's marriage to Apollonia occurring entirely in Sicily, Kay is apparently unaware that Michael was a widower when he married her in the USA.

Kay is still very isolated from the decisions Michael makes as the new Don. At the beginning of The Godfather, Part II (set in 1958-59), Kay, who is pregnant, implores Michael to fulfill his promise of legitimizing the family business. Michael makes a sincere effort to break the family's criminal ties, but his escalating war with rival Hyman Roth (Lee Strasberg) keep him trapped in the criminal underworld and in his personal obsession with revenge. During one of Michael's trips, Kay has an abortion, fearing that another male heir would tie the family to the Mafia forever. While she initially tells Michael that it was a miscarriage, she is increasingly repulsed by the campaign of murder and terror he wages in his war with Roth, and she reveals the truth to him during an argument. Enraged, he banishes her from the family; the two are soon divorced.

In the 2004 novel The Godfather Returns, a retcon occurs regarding Kay's termination of her pregnancy. Kay suspects Michael of having their family doctor murdered (as he ostensibly believed that this doctor was responsible for the abortion). Kay confronts Michael and confesses that she had lied; it was, in fact, a miscarriage (due to stress brought on by neglect), and she had lied to Michael in order to hurt him.

In The Godfather, Part III (set in 1979-80), Kay and Michael have not been in contact for several years. Michael has extracted himself from criminal enterprises and has even sold the casinos, and he has been given a knighthood by the Pope, in recognition of his charitable work. After an uneasy reunion, the two reconcile their differences and begin to rekindle their relationship after Michael retires and appoints his nephew Vincent (Andy Garcia) the new Don. Michael reluctantly consents to permit their son, Tony, to become an opera singer. (Conveniently, he is a tenor. They plan to see his operatic debut, in Palermo.) Just as they begin a new life together, however, their daughter, Mary (Sofia Coppola), is killed in an assassination attempt on Michael. This tragedy breaks Michael's spirit, and he withdraws from Kay and from life itself until his lonely death in 1997.