Demoncy joined the youth academy of Paris Saint-Germain, and since 2014 played for their reserve side - eventually becoming captain of the reserves.[2] On 25 June 2016, he signed his first professional contract with PSG for three years.[3]
Orléans
On 30 January 2018, Demoncy joined US Orléans on loan from Paris Saint-Germain for the second half of the 2017–18 season.[4] He made his professional debut with Orléans on 9 February 2018, scoring the late winner in extra time in a 3–2 Ligue 2 win over Chamois Niortais[5]
On 22 June 2018, he joined Orléans permanently on a three-year deal.[6]
Paris FC
In July 2021, Demoncy joined Paris FC on a three-year deal.[7]