Cvetic was born in 1909 to Slovenian immigrants living in Pittsburgh. One of 11 children, Cvetic graduated from St. Mary's Roman Catholic Grammar School in 1922. Thereafter, his formal education is difficult to verify, in part due to the often-contradictory testimony he provided before various audiences over the course of his career. At various points he claimed to have attended "prep school or college," although during a hostile cross examination in 1954 was forced to admit that he had only completed the tenth grade. Before his employment with the Federal Bureau of Investigation, Cvetic worked a series of jobs, including various sales positions as well as briefly serving at the Pennsylvania State Department of Labor and Industry. On August 15, 1931, he married a pregnant Marie Barsh, who gave birth to twin sons the next year. The marriage proved to be a tumultuous one, as Barsh claimed that he was physically and verbally abusive with her as well as unfaithful, leading to a separation followed by a divorce in January, 1946. This period was especially difficult for Cvetic as he was arrested in 1939 after allegedly having beaten his sister-in-law badly enough that she required hospitalization. During World War II, he volunteered to join the United States Army but was rejected for being too short.