Mercier was born in Chaudefonds-sur-Layon, France, and ordained a priest of the Diocese of Angers on 27 June 1970.

From 1971 to 1974 he completed his training at the Pontifical Gregorian University in Rome where he obtained a license and then a doctorate in canon law.

He was chaplain of the College and Catholic High School in Angers from 1979 to 1987, when Bishop Jean Orchampt made him his private secretary.

In January 2002 he entered the service of the Roman Curia as official of the Congregation for Bishops.

On 8 January 2015, Pope Francis appointed him Secretary of the Congregation for the Clergy and named him titular Archbishop of Rota in Spain.