He graduated with distinction in 1943, then went to serve with the New Zealand Medical Corps at World War II in the Middle East, Italy and Japan.
Upon returning to New Zealand he worked in medicine in Dunedin and Wellington Hospitals before travelling to Britain where he was a student and house physician at the Royal Post Graduate Medical School at Hammersmith.
He graduated from Hammersmith in 1948, and received a Nuffield Fellowship which led to his appointment to the National Hospital for Nervous Diseases, Queen Square, London.
Bergin was devoutly committed to the Catholic faith, described by Haas and Hornabrook as "able to articulate, intellectualise, and simplify aspects of Catholicism, and his arguments were always persuasive".
[1] For over forty years he was an active member of the Catholic Doctors' Guild of St Luke, SS Cosmas, & Damian which he also served as Master.