Susana Chavez was found strangled and with one hand cut off in Ciudad Juarez last week, but has only now been identified.
Ms Chavez tried to draw attention to the killing of mainly poor women in the border town in the 1990s.
Officials say her murder was not related to her activism.
The Chihuahua State Attorney General's Office said she was killed by three teenagers high on drugs, who cut off her hand to make it look like the murder was connected to organised crime.
Wave of killings
Ms Chavez, 36, coined the slogan "Not One More Death", which became popular at protests against the Ciudad Juarez killings and the failure of the police to solve them.
More than 300 women were murdered in Ciudad Juarez in a wave of violence which started in 1993 and lasted for a decade.
There is no generally accepted motive for the murders.
They have been variously attributed to serial killers, drug cartels and domestic violence. Some of the killings are believed to have been sexually motivated.
Ms Chavez was active in an organisation called May Our Daughters Return Home, which represents the families and friends of the killed women.
But Attorney General for Chihuahua State Carlos Manuel Salas says her death was the result of an "unfortunate encounter" with the teenagers, who got involved in an argument with Ms Chavez and strangled her.
Human rights group Amnesty International said that although her murder did not seem to be related to her activisim, Ms Chavez's killing was another sign that violence against women was again on the rise in Ciudad Juarez.
Ciudad Juarez is the most violent city in Mexico, with 3,100 people killed in 2010 out of a population of more than a million.