He entered the Clerics Regular of Somasca, more commonly known as the Somascan Fathers, in 1924, and made his profession on 3 October 1930.

He was ordained to the priesthood on 19 July 1936, and then did pastoral work at La Ceiba Institute in San Salvador until 1948, whence he became its rector.

He received his episcopal consecration on the following 27 December from Pope John himself, with Bishops Girolamo Bortignon, OFM Cap, and Gioacchino Muccin serving as co-consecrators, in St. Peter's Basilica.

Casariego was kidnapped for several days by a terrorist group (right extremists from a death squad who wanted to blame Guatemalan leftist guerrilla) in March 1968.

The Cardinal was an ardent supporter of Guatemala's authoritarian regime,[2] so much so that his automobile was accompanied by a radio patrol and two armed motorcycle guards.