1970 - He served in the US Army before joining the FBI.
1980 - Ressler helped to organize the interviews of thirty-six incarcerated serial killers in order to find parallels between such criminal's backgrounds and motives.
- This consists of a centralized computer database of information on unsolved homicides.
1990 - Ressler retired from the FBI and is the author of a number of books about serial murder.
1993 - He was brought in to London to assist in the investigation into the murders committed by Colin Ireland.
- John E. Douglas, whom Ressler was a mentor to at the FBI's Behavioral Analysis Unit, as well as a colleague and friend.