He received a Bachelor of Science degree in physics from University of North Carolina and a doctorate in medicine, also from UNC.

He returned to the University of North Carolina Medical School in 1959, graduated in 1963, and completed internship training in 1964 at the Wilford Hall USAF Hospital at Lackland Air Force Base, San Antonio, Texas.

[2] Thornton returned to active duty with the United States Air Force and was then assigned to the USAF Aerospace Medical Division, Brooks Air Force Base, San Antonio, where he completed the Primary Flight Surgeon's training in 1964.

It was during his two-year tour of duty there that he became involved in space medicine research and subsequently applied and was selected for astronaut training.

Thornton was physician crew member on the highly successful Skylab Medical Experiments Altitude Test (SMEAT)—a 56-day simulation of a Skylab mission enabling crewmen to collect medical experiments baseline data and evaluate equipment, operations, and procedures.

He developed advanced techniques for, and made studies in, kinesiology and kinesimetry related to space operations.

[2] During Space Shuttle operations he continued physiological investigations in the cardiovascular and musculoskeletal and neurological areas.

[2] Thornton holds more than 60 patents that range from military weapons systems through the first real-time EKG computer analysis.

Space-related items include the first in-flight mass measurement devices, shock and vibration isolation systems, an improved waste collection system, an improved lower body negative pressure (LBNP) apparatus, and others.

The mission was accomplished in 98 orbits of the Earth, traveling 2.2 million miles in 145 hours, 8 minutes, 4 seconds.

The Spacelab-3 science mission was launched from Kennedy Space Center, Florida, and returned for a night landing at Edwards Air Force Base, California.