Here, he defended his thesis, titled Le dépérissement de l’État selon Karl Marx.

He was a diaconate in the Diocese of Coutances from 1990 to 1996, and was one of the auditors at the Institut des hautes études de défense nationale from 1993 to 1994.

He was appointed as Bishop of Clermont on 22 February 1996 and took office on 4 May, succeeding Jacques Fihey and residing at the Clermont-Ferrand Cathedral.

His position was elevated to archbishop status on 16 December 2002 following a redistribution of ecclesiastical provinces in France by Pope John Paul II.

Simon represented France in the Commission of the Bishops' Conferences of the European Community until November 2007, when he was succeeded by Christian Katz of Strasbourg.