She was born in Mornese, to a peasant family who worked in a vineyard. She was the eldest of ten children of Joseph and Maddalena Calcagno Mazzarelli. When she was fifteen she joined the Association of the Daughters of Mary Immaculate, run by her parish priest, Father Domenico Pestarino; it was a precursor to the founding of the Salesian Sisters.
The Daughters were known for their charitable works and Mary soon set herself apart for her sound judgment, dedication, joy, and love of the young. Wherever she went the village children were drawn to her like a magnet, eager to hear her stories, or to ask her a multitude of questions about the Christian faith.
When she was 23 a typhoid epidemic hit Mornese and villagers started rapidly dying. Soon her uncle and aunt were taken ill and Mary volunteered to care for them and their many children. After a week they were healed, but when she returned home Mary became ill with typhoid herself. She received the last rites of the Church and recovered, but the illness left her weak. The strength which had formerly sustained her in the fields was no more. Mary was now thin and frail; a shell of her formerly robust self.
Her practicality led her to find other means to sustain herself, so she took an apprenticeship as a seamstress in the town and worked diligently at the craft. Like John Bosco, the skills which she learned in her youth she was later able to pass onto those who would come after her. After she recovered from her illness, in the month of October, Mary was walking in her village and was suddenly astounded to see before her a large building with a courtyard and many girls playing and laughing. A voice said to her, “I entrust them to you.”
At the same time St John Bosco had a similar experience where he was shown a group of girls abandoned in a courtyard. The same voice said to him, “These are my daughters; take care of them.” Mary was to become the co-founder of a religious order to care for young girls, just as the Salesian priest and brothers cared for young boys.