Brenner, who was born Mary Clarke, died of natural causes Thursday after being in declining health, her daughter-in-law, Christina Brenner said.
The mother of seven had gone through two divorces when she decided in 1977 to give away her belongings, become a Roman Catholic nun and move into the La Mesa penitentiary to work, the Los Angeles Times said.
She had previously delivered donations to the prison and said she was filled with compassion for the 8,000 or so men in the prison.
"Something happened to me when I saw men behind bars. ... When I left, I thought a lot about the men. When it was cold, I wondered if the men were warm; when it was raining, if they had shelter," Brenner told the Times in a 1982 interview. "I wondered if they had medicine and how their families were doing. ... You know, when I returned to the prison to live, I felt as if I'd come home."
Brenner eventually established her own religious order in Tijuana, the Eudist Servants of the Eleventh Hour, the Times said.
She is survived by her seven children and 45 grandchildren and great-grandchildren, the newspaper said.