She was the youngest but one of a very large family. Her father, Giacomo di Benincasa, was a dyer; her mother, Lapa, the daughter of a local poet. They belonged to the lower middle-class faction of tradesmen and petty notaries, known as "the Party of the Twelve", which between one revolution and another ruled the Republic of Siena from 1355 to 1368.
From her earliest childhood Catherine began to see visions and to practice extreme austerities. At the age of seven she consecrated her virginity to Christ; in her sixteenth year she took the habit of the Dominican Tertiaries, and renewed the life of the anchorites of the desert in a little room in her father's house. After three years of celestial visitations and familiar conversation with Christ, she underwent the mystical experience known as the "spiritual espousals", probably during the carnival of 1366. She now rejoined her family, began to tend the sick, especially those afflicted with the most repulsive diseases, to serve the poor, and to labour for the conversion of sinners.
Though always suffering terrible physical pain, living for long intervals on practically no food save the Blessed Sacrament, she was ever radiantly happy and full of practical wisdom no less than the highest spiritual insight. All her contemporaries bear witness to her extraordinary personal charm, which prevailed over the continual persecution to which she was subjected even by the friars of her own order and by her sisters in religion. She began to gather disciples round her, both men and women, who formed a wonderful spiritual fellowship, united to her by the bonds of mystical love.
During the summer of 1370 she received a series of special manifestations of Divine mysteries, which culminated in a prolonged trance, a kind of mystical death, in which she had a vision of Hell, Purgatory, and Heaven, and heard a Divine command to leave her cell and enter the public life of the world. She began to dispatch letters to men and women in every condition of life, entered into correspondence with the princes and republics of Italy, was consulted by the papal legates about the affairs of the Church, and set herself to heal the wounds of her native land by staying the fury of civil war and the ravages of faction. She implored the pope, Gregory XI, to leave Avignon, to reform the clergy and the administration of the Papal States, and ardently threw herself into his design for a crusade, in the hopes of uniting the powers of Christendom against the infidels, and restoring peace to Italy by delivering her from the wandering companies of mercenary soldiers.
While at Pisa, on the fourth Sunday of Lent, 1375, she received the Stigmata, although, at her special prayer, the marks did not appear outwardly in her body while she lived.