Mrs Callejas was married to a convicted American assassin, Michael Townley, who worked for the US-backed Pinochet government in the mid-seventies.
He was jailed over the murder of two Pinochet opponents, General Carlos Prats and a former foreign minister, Orlando Letelier.
She was later convicted for her role in the murder of the general.
She was famed for her literary evenings at her house in Santiago, allegedly while intelligence officers tortured political prisoners in the basement.
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The former head of Chile's feared intelligence service Dina, General Manuel Contreras, while giving evidence at his trial, had said the Mariana Callejas had set the bomb off remotely that killed General Prats and his wife in Buenos Aires.
She was given a 20-year jail sentence in 2008 but the Chilean Supreme Court reduced this afterwards to only five years under house arrest.
A journalist, John Dinges, who has written extensively about the history of the Pinochet era said: "I definitely think Callejas was excited by all this, and it was a big adventure for her.
"She was entirely without scruples. She didn't seem to do it for ideological reasons."
In 1980 she published a collection of stories under the title The Long Night, in which she described torture sessions and building bombs.
This year she won a mention in a prestigious Latin America literary prize for one of her stories.
A Chilean writer and former government secret agent, Mariana Callejas, has died at the age of 84.