McCool was born in San Diego, California. His family moved to Lubbock, Texas where he graduated from Coronado High School in 1979. He received his bachelor of science degree in applied science from the United States Naval Academy in 1983, then a master of science degree in computer science from the University of Maryland in 1985, and a master of science degree in aeronautical engineering from the US Naval Postgraduate School in 1992.
McCool was selected by NASA and reported to the Johnson Space Center to became an astronaut in 1996.
After his death, Asteroid 51829 Williemccool was named for him.