Born in Cà Mau, Vietnam, Asia, Mẫn studied in Cần Thơ and Saigon, and was ordained a priest on 25 May 1965, serving the diocese of Cần Thơ.
Lahir di Cà Mau, Vietnam, Asia, Mẫn belajar di Cần Thơ dan Saigon, dan diangkat menjadi imam pada 25 Mei 1965 dan bertugas di keuskupan Cần Thơ.