Born in Santiago, second of the six children of Pedro Errázuriz Larraín and Marta Ossa Ruiz. He studied at the Liceo Alemán de los padres del Verbo Divino and later at the Faculty of Engineering of the Pontifical Catholic University of Chile, where he graduated with a bachelor degree in mathematics in 1953. For 8 years, he was a member of the Student Center and of Federation of Students and joined different university groups of the nascent Schönstatt Movement. Errázuriz befriended the Servant of God Mario Hiriart and with him and other students originated this Marian apostolic movement in Chile.