His mother Catharina née Noelege was a professional pianist and had been a patient teacher to Piet such that he was able to be admitted to primary school without having to attend kindergarten due to his reading and writing skills.

Piet was raised in a Roman Catholic family, and while attending the Canisius College Mater Dei gymnasium he became interested in becoming a priest, specifically as a member of the Society of Jesus.

Father J. Willekens, S.J., his mentor at the novitiate, advised Zoetmulder to consider working for the mission on the island of Java.

In 1931 Zoetmulder graduated cum laude and was made a candidate priest in Girisonta, Ungaran, near Semarang.

In October 1935 he successfully defended his doctoral thesis "Pantheïsme en Monisme in de Javaansche Soeloek Literatuur," with the help of Berg.

After he arrived in Jakarta, Zoetmulder was offered a post in the linguistics department of the University of Indonesia.

When he was transferred to the Cimahi prison he was able to smuggle the Adiparwa edited by Dr. H. H. Juynboll [nl] (Âdiparwa; Hendrik Herman Juynboll, ed., 1906 Koninklijk Instituut voor Taal-, Land- en Volkenkunde, M. Nijhoff) and a Javanese dictionary written by Johann Friedrich Carl Gericke [id]-Taco Roorda [nl].

His study of old Javanese grammar was eventually published in Dutch in 1954 ("De Taal van het Adiparwa") and in Indonesian in 1950 ("Bahasa Parwa") with the help of I. R. Pedjawijatna.

During the Indonesian National Revolution, he was nearly killed by a freedom fighter due to his Dutch complexion while he was at the Kemetiran Residence, in Yogyakarta.

Five years later he was made a Distinguished Permanent Professor of Pedagogic Literature at the same university, by a mandate from the Minister of Education and Culture.

He also wrote a compendium entitled "Sekar Sumawur: Bunga Rampai Bahasa Djawa Kuno".

His book "Kalangwan", containing an account of old Javanese belles lettres and writers, was published in 1974, to be followed by the "Old Javanese-English Dictionary" in 1982.

He admitted difficulty in writing due to his having to collect his material from microfilm located at the University of Leiden.

In addition to his religious and scientific textbooks, he also devoured volumes of novels poetry, and detective series.

Le Carré's other story "A Small Town in Germany," was "inspected" by Father Zoet before publication.