Castellanos was born in San Francisco de Becerra, Olancho on 8 January 1954. Her mother was Rafael Castellanos of Santa Bárbara, and her father was Ernestina Ruiz of Olancho. She grew up in the sugar fields of rural Honduras. In 1968, Castellanos' father gave her a test to be a student at the Normal School for Girls in the city of Tegucigalpa. She entered the school and graduated in 1973 with a teaching degree. In 1974, Castellanos won two scholarships to study social work at the National Autonomous University of Honduras (UNAH) and social sciences in the College of Teachers. After several years of study, she finished her studies with a master's degree in economics from UNAH and a bachelor's degree in sociology from the University of Costa Rica. 