Lasuén was born at Vitoria in Álava, Spain, on 7 July 1736 and joined the Franciscan order as a teenager, entering the Friary of San Francisco shortly before his fifteenth birthday on 19 March 1751.

"[7] It is clear from his diaries that Lasuén struggled with loneliness and perhaps some depression brought about by the extreme conditions he encountered in San Diego when he was asked to return to restore order after the murder of Fray Jayme.

Lasuén was fearful of Indian uprisings, and often wrote Serra and Felipe de Neve, Governor of California (1777 to 1782) about his difficulties, seeking advice about the placement of both friaries and the number of troops to guard the mission properties.

[10] His Christian zeal and sense of "civilizing" purpose led him to great lengths in order to acculturate Native Americans, even using their language in his pursuit, despite the Spanish king's prohibition in that respect.

News of the mistreatment of Native Americans in the Mission of San Francisco reached the governor of California Diego de Borica, also a Basque, who warned of a lawsuit against Lasuén should he not give up on his practices.

[11] Although it is clear that, at points during their ministry and friendship, they did not always agree about how to discipline the Indians and protect mission property, Lasuén wrote that Serra was a "most exemplary superior and a holy man.