[4] Ressler joined the FBI in 1970 and was recruited into the Behavioral Science Unit, which deals with drawing up psychological profiles of violent offenders, such as rapists and serial killers, who typically select victims at random.

[citation needed] He worked on many cases of serial homicide such as Jeffrey Dahmer, Ted Bundy, Richard Chase and John Joubert, and Montie Rissell.

He actively gave lectures to students and police forces on the subject of criminology and, in 1993, was brought in, in London, to assist in the investigation into the murders committed by Colin Ireland.

In 1995, Ressler met South African profiler Micki Pistorius at a conference in Scotland and she invited him to review her investigation of the "ABC Murders", so-called because of their location in the Johannesburg suburbs of Atteridgeville, Boksburg, and Cleveland.

As predicted, serial killer Moses Sithole called the South African newspaper The Star to claim responsibility for the Atteridgeville and Boksburg murders, some time after Ressler left the case.

[7][8] Ressler's visit to Ciudad Juárez in Mexico to investigate the still-active feminicides occurring there served as inspiration for the character Albert Kessler in Roberto Bolaño's novel 2666.

[10] A screenplay adapted from his colleague John E. Douglas' book Mindhunter: Inside the FBI's Elite Serial Crime Unit was picked up by Netflix.