Born in Orsay, France, Demel started his professional career in the French Ligue 2 with Nîmes Olympique. He was signed by Arsenal in August 2000 spending one season in London without making a first team appearance before being transferred to Borussia Dortmund. He made his Bundesliga debut in 2003 winning the Bundesliga and making UEFA Cup Final in his first season. Demel left Dortmund to join Hamburg in 2005, after struggling to hold down a first team place. With Hamburg, Demel played regularly as both a right back and a midfielder. In May 2006, he signed a new contract with Hamburg, tying him to the club until 2010. Demel was released by Hamburg at the expiry of his contract in 2011. He made 199 appearances for Hamburg including 52 in European football in the UEFA Cup, UEFA Europa League and the 2006–07 UEFA Champions League, scoring four goals.
West Ham United
He signed a two-year deal, with the option of a third, with West Ham on 31 August 2011. Due to injuries it was not until 29 November 2011, in a 2–0 away win against Middlesbrough, that Demel made his West Ham United debut. With West Ham playing in the Championship, Demel played 10 games in the 2011–12 season, eight of them after coming on as substitute. On 19 May 2012, he was a member of the team which won promotion to the Premier League after beating Blackpool in the 2012 Football League Championship play-off Final at Wembley Stadium. He played the first 57 minutes before being replaced by Julien Faubert. On 23 January 2013, Demel extended his contract with West Ham until 2015. Demel scored his only West Ham goal in a 2–1 home defeat by Liverpool on 6 April 2014. After West Ham striker, Andy Carroll challenged Liverpool goalkeeper, Simon Mignolet, the goalkeeper dropped the ball and Demel scored from out. Demel was first-choice right-back for the 2012–13 and 2013–14 seasons playing 31 and 32 games respectively. By the 2014–15 season, Carl Jenkinson was first-choice and Demel managed only six games all season. He was released at the end of the 2014–15 season.
Dundee United
In November 2015, Demel signed for Dundee United until the end of the 2015–16 season. He scored his first goal for Dundee United in a 2–1 win over Hearts on 20 February 2016. Following confirmation of the club's relegation from the Scottish Premiership, Demel was allowed to leave the club and return to France before the end of the season, despite still being under contract.
