- "I did everything with Sonny."
- ―Lucy Mancini[src]
- "Santino, come here. What's the matter with you? I think your brain is going soft from all that comedy you're playing with that young girl."
- ―Vito Corleone referring to Lucy[src]
The daughter of Jimmy Mancini, a lifelong soldato of Vito Corleone, Lucy was one of the childhood friends of Vito's children, particularly his daughter, Connie. She was the maid of honor at Connie's wedding to Carlo Rizzi in 1945. Lucy had sex with Sonny at the wedding, beginning a years long extramarital affair with him. This affair eventually resulted in the birth of Vincent Mancini, who would later become the Don of the Corleone Family. This affair caused estrangement between her and Connie. The Family often used her apartment in Midtown as a safehouse.
After Sonny's murder in 1948, Lucy was sent to Las Vegas by Vito's consigliere, Tom Hagen, following a failed suicide attempt due to her grief. There she was given a small interest (five and later ten "points") and a job in one of the family's hotels, primarily so that she could keep an eye on Fredo Corleone, who was learning the hotel business from Moe Greene. She also served as a shareholder-of-record, as she had no criminal record. Lucy realized that she was being used, but she did not mind as she was grateful for the second chance.
Eventually, she established a whole new life for herself in Las Vegas, and became largely independent of the Corleones. She was very lonely, however, and occasionally pined for Sonny: while she did not love him or even really know him, she missed him as a lover, and couldn't achieve sexual satisfaction with anyone else. That changed when she met and befriended a surgeon, Dr. Jules Segal, who explained that her difficulty in reaching orgasm was caused by a loose vagina — which commonly results from multiple childbirths but in Lucy's case was congenital — and could be fixed with simple surgery. Lucy agrees to the surgery and a colleague in Los Angeles performs the operation, Lucy was finally able to enjoy sex with Jules and he proposed to her that very night. While in Las Vegas, she also reconnected with friends from her childhood in New York, now successful actors Johnny Fontane and Nino Valenti. Later that year, Lucy attended the funeral of Vito Corleone with Fredo.
After Las Vegas
- "Vincent, our name’s not on the list."
- ―Lucy Mancini[src]
- "Write it in. Mancini: M-A-N-C-I-N-I."
- ―Vincent Mancini[src]
Lucy's happy life was shattered when Segal was murdered several years later under orders from Michael Corleone, who falsely suspected he had a hand in Kay's abortion. Following this, Lucy likely moved back to New York with Vincent, who would later enter the Corleone "family business".
She appeared at the party celebrating Michael's papal honor with her son, and though they were not on the guest list, they were recognized and let in. Lucy seemed proud to see her son join the Corleone family picture.