Onizuka received a Bachelor's degree in aerospace engineering in June 1969, and a Master's in that field in December of the same year, from the University of Colorado. He then entered active duty with the United States Air Force, where he served as a flight test engineer and as a test pilot.
He had been selected for the astronaut program in January 1978, and had flown a previous mission on the Space Shuttle Discovery in January, 1985, also serving as a mission specialist. He held the rank of Lieutenant Colonel at the time of his death.