He was born in Santiago, Chile, the sixth of 15 children of Juan Francisco Larraín Rojas and of Mercedes Gandarillas Aránguiz.

Before entering the priesthood, the young Joaquín had written several impassioned articles dealing with apologetics and other religious subjects, in catholic magazines.

His priesthood and subsequent episcopate were characterized by the untiring energy and initiative that he applied on all his missions and in the exercise of his ministry.

He was the first president ("rector") of the Pontifical Catholic University of Chile, which was founded June 21, 1888 through a decree issued by the Santiago Archbishopric.

In 1864 Larraín Gandarillas was elected as a member of parliament (diputado) for the (now extinct) electoral district of Rere, near Concepción.