She was one of the leaders of the Manuel Rodríguez Patriotic Front (FPMR), a guerilla organization which arose from the Communist Party of Chile in the 1980s to fight against the military dictatorship of Augusto Pinochet.

[1][2] In the mid-1970s, while studying sociology at the University of Chile, she began to identify with the struggle of those opposed to the military dictatorship of Augusto Pinochet, and took part in numerous student demonstrations.

Her work was centered in Santiago de Chile and Rancagua,[3] cities where she recruited new members for the organization and provided logistical support for the emerging combat groups in those regions.

In mid-1986, "Commander Tamara" was given the responsibility of leading one of the riskiest actions undertaken by the FPMR up to that point: the attempted assassination of Augusto Pinochet, also known as Operation 20th Century (Spanish: Operación Siglo XX).

During the mission, "Tamara" acted as the right hand of the top leader of the attack, José Joaquín Valenzuela Levi, also known as "Commander Ernesto".

In late August,[5] she and fellow FPMR member César Bunster rented a house and three vehicles, and coordinating the transfer of the weapons that would be used in the ambush.

Initial plans had Magni acting s a riflewoman in the operation, but she was pulled from the role at the last minute, due to the high probability that the combatants would not come out alive.