Daniel Billen was born in New York City, New York on November 27, 1924. He earned his BS (1948) from Cornell University and his MS (1949) and PhD (1951) from the University of Tennessee. Billen began his professional career as an Instructor of Bacteriology at the University of Tennessee (1950-1951) and went on to work as a Biologist at the Oak Ridge National Laboratory (1951-1957) before accepting a position with the M. D. Anderson Hospital and Tumor Institute in Houston, Texas. Here, he worked as an Associate Biologist and Microbiology Section Chief (1957-1961) and as a Biologist and Radiation Biology Section Chief (1961-1966) in addition to heading the National Science Foundation's Metabolic Biology Program (1960-1961), serving on the University of Texas Biomedical Sciences Graduate School's graduate faculty (1960-1966), teaching Biology at the University of Texas (1965-1966), and helping to organize the Radiation Protection and Recovery Conference (1964). He became Head of the Radiation Biology Laboratory and Professor of Radiation Biology at the University of Florida in 1966, where he remained until accepting the directorship of the University of Tennessee-Oak Ridge Graduate School of Biomedical Sciences in 1974.

Billen married Eleanor Berlin on March 2, 1951 and the couple had three children: Jerome (1955-), Rhonda (1953-), and Robin (1961-).