Leaving the Divine Word Missionaries to serve the Diocese of Tuguegarao, Lazo was ordained to the priesthood by Archbishop Mariano Madriaga on March 22, 1947. He then served as an assistant priest at St. Peter's Cathedral in Vigan, and later at San Jose parish in Baggao. He also founded San Jose Academy. In 1950, Lazo was named prefect of discipline at the newly created San Jacinto Minor Seminary, of which he later became rector in 1951.
He became parish priest of Lal-Lo in 1967 and, due to the large number of students, founded the Lyceum of Lal-Lo in 1968.
In 1998, he made a Declaration of Faith to John Paul II, saying "obedience must serve faith".
In the same document, Lazo expressed skepticism of the reforms of the Second Vatican Council, saying "If the Conciliar reforms are according to the will of Jesus Christ, then, I will gladly cooperate in their implementation. But if the Conciliar reforms are planned for the destruction of the Catholic Religion founded by Jesus Christ, then, I refuse to give my cooperation."
In his later autobiography however, Lazo viewed the post-Conciliar reforms as "Masonic inspired," and "aimed to destroy the Catholic Religion." Lazo also said that, apart from Satan and Freemasonry, he identified "Talmudic Judaism" as one of the "three enemies of the Catholic Church".