Roy Hazelwood was born in Pocatello, Idaho. When he was an infant, his biological father, Myrle Reddick, kidnapped him and travelled around with him for six months before returning the boy to his paternal grandparents; father and son never saw each other again. He was raised by his mother and stepfather in Spring Branch, Texas, and attended Sam Houston State University.

He served a tour in Vietnam, which he completed in 1968 as a Major. He followed his tour with a forensic medicine fellowship with the Armed Forces Institute of Pathology (AFIP) and a stint with the CID as an instructor.

He joined the FBI in 1971.

In 1980, he developed the distinction between "organized" and "disorganized" murderers, a concept that is still used by law enforcement to help in the apprehension of criminals. He also defined the six categories of rapists: power-reassurance, power assertive, anger retaliatory, anger excitation, opportunistic and gang. Of the six, anger excitation is by far the most dangerous and the hardest to capture.

Hazelwood has also offered the theory that there is no cure for pedophilia or sexual sadists. He has conducted numerous studies involving sex crimes, including cases of autoerotic asphyxiation. He has done numerous studies involving the willing victims of sexual sadists (wives and girlfriends) and how sexual sadists appear in everyday life. In his career he found equivocal death crime as the most dubious and complex investigation to overcome.

Hazelwood, now retired from the FBI, is an active member of the Academy Group, Academy Group, Inc. - Roy Hazelwood (Robert R. Hazelwood, M.S.) an organization of former FBI agents and law enforcement officers. He continues to work closely with the FBI and other government agencies in an effort to track down sexually-oriented murderers. Hazelwood has also co-authored two books with Stephen Michaud: The Evil That Men Do and Dark Dreams.

Hazelwood now gives lectures across North America about sexual sadism and autoerotic fatalities. His presentation is also heavily focused on Dennis Rader, the "B.T.K." serial killer of Wichita.

Hazelwood, a devout Presbyterian, is married with three grown children.