Reilly was born at Mountain Home Air Force Base, Idaho, on March 18, 1954, though considers his hometown to be Mesquite, Texas.

[5] During graduate school, Reilly was selected to participate in the 1977–1978 scientific expedition to Marie Byrd Land, West Antarctica, as a research scientist specializing in stable isotope geochronology.

He reported to the Johnson Space Center in March 1995 and completed a year of training and evaluation, and qualified for flight assignment as a mission specialist.

[9] After leaving government service in 2020, Reilly later joined defense contractor Booz Allen Hamilton the following year as an executive adviser.

[11][12] His statements on the National Climate Assessment focused on scientific uncertainties rather than directly stating concurrence with opinions expressed by some scientists.