Ordained a priest in 1960, Franck's first assignment was as associate pastor at St. Joseph Parish in Differdange, where he served until 1971.

He then became pastor at St. Cuningundis Parish in Clausen, and also served as National Director for the Catholic Action of Children in Luxembourg and the Pontifical Mission Aid Society, both 1969–1977.

In conjunction with these offices, he was also named Ecclesiastical Counsellor at the Embassy of Luxembourg at the Holy See (from 1981)[1] and was a member of the Pontifical Council for Social Communications at the Vatican.

[1] His motto Ut unum sint, (English: that they may be one, (John 17: 11) has inspired his pastoral leadership and his constant dedication to both the local and central Church.

[1] Sacred Heart University, which operates an overseas campus in Luxembourg, recognised his outstanding leadership and his ongoing support of the laity in the Church by conferring on him an honorary doctorate in humane letters in 2003.