Marcoux studied at the Université Laval and obtained a bachelor's degree in arts as well as a licence in law in 1963 and obtained another diploma in law in 1996.
He then worked for the Ministry of Finance and the Quebec Treasury Board.
Prior to his political career, Marcoux held several important administration positions including vice-president for the National Bank, Laurentian Bank and Trust La Laurentienne, in the early 1980s as well as key positions at the province-run "Société général de financement du Quebec" (SGF) as well as private supermarket Provigo.
Before his jump to politics he was the general manager of the "Association des Hopitaux du Quebec".
Marcoux won in the 1998 elections the Vaudreuil seat which was formerly held by Daniel Johnson Jr. former Quebec Premier.