Dee was born Alexandra Zuck on April 23, 1942, in Bayonne, New Jersey, the only child of John Zuck and Mary (née Cymboliak) Zuck, who met as teenagers at a Russian Orthodox church dance. They married shortly afterward, but divorced before Sandra was five years old. She was of Carpatho-Rusyn ancestry, and raised in the Russian Orthodox faith.

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Her son, Dodd Darin, wrote in his biographical book about his parents, Dream Lovers, that Dee's mother, Mary, and her aunt Olga "were first generation daughters of a working-class Russian Orthodox couple." Dee recalled, "we belonged to a Russian Orthodox Church, and there was dancing at the social events." Alexandra would soon take the name Sandra Dee. She became a professional model by the age of 4 and subsequently progressed to television commercials.

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There has been some dispute as to Dee's actual birth year, with evidence pointing to both 1942 and 1944. Legal records, including her California divorce record from Bobby Darin, as well as the Social Security Death Index (SSDI) and her own gravestone all give her year of birth as 1942. In a 1967 interview with the Oxnard Press-Courier, she acknowledged being 18 in 1960 when she first met Bobby Darin, and the couple wed three months later. According to her son's book, Dee was born in 1944, but, having begun modelling and acting at a very young age, she and her mother falsely inflated her age by two years so she could find more work.