[4] He was ordained a priest on 25 May 1965 by Bishop Jacques Nguyễn Ngọc Quang, for Diocese of Cần Thơ.

[6] In 1993, he was appointed Coadjutor Bishop of Mỹ Tho and promoted to Archbishop of Hồ Chí Minh City in 1998.

He officiated at a mass in La Vang in 1998 for the 200th anniversary of the apparition of the Virgin Mary, the largest Catholic event in Vietnam up to that time.

On 7 March 2013, Mẫn was the last of the cardinal electors to arrive to attend the 2013 papal conclave,[8] which subsequently elected Pope Francis.

[9] On 22 March 2014, Pope Francis accepted the resignation of Cardinal Mẫn due to age limit.