Born in London, Demontagnac started his playing career with Charlton Athletic, before moving to join the Walsall youth system.

Demontagnac created an instant impression on the Walsall coaching staff with his fast pace and footballing talents, especially in getting past defenders.

[4] He went on to make 27 appearances that season, scoring twice, and picked up the club's Young Player of the Year award,[3][5] but was twice sent off for violent conduct.

Money selected Demontagnac for the starting line-up just once in the season, choosing to use him as an impact player from the substitutes' bench.

[3] Following a defeat against Crewe Alexandra, new manager Jimmy Mullen recalled him to the first team and handed him a start for The Saddlers against Southend United.

[11] Mullen expressed regret over the situation, saying that Demontagnac had the talent to be a Premier League player but his poor conduct held him back.

[12] The trial was unsuccessful, but Palace manager Neil Warnock allowed Demontagnac to train and play with the reserve team in the meanwhile.

[18] On 8 August, he was an unused substitute as the Seasiders started the 2009–10 season with a 1–1 draw with Queens Park Rangers at Loftus Road.

[19] Four days later he made his league debut, and his 100th career appearance, as a second-half substitute in a 1–1 draw with Cardiff City at Bloomfield Road.

[21] He made his return to action in a 4–3 Central League Division One West reserve team win over Preston North End on 14 October.

[30] After his release from Notts County, Demontagnac signed a two-year contract with Northampton Town and joined them on 1 July 2012.