Jason Demetriou made his league debut for Leyton Orient during the 2005/6 season aged 18 in the clash with Cheltenham. He was at the Norwich City Centre of Excellence at Potters Bar along with brother Danny in the late 1990s until a change of rules by the Football Association Forced Norwich to close the centre due to travelling times. Jason then spent four years with Arsenal and a year with Tottenham before joining Orient.

Following Orient's promotion to League One at the end of the 2005/6 season, Jason was offered a professional contract by the O's

On 21st September 2008, the News of the World reported that Leyton Orient midfielder Jason Demetriou, 20, is a target for both Newcastle and Charlton

He made his debut for Cyprus in February 2009 as a substitute in a 2-0 defeat against Serbia. He made his full debut later that month assiing with two of Cypr' goals in their 3-2 win over Slovakia.

Rumours had it in July 2009 that Championship side Cardiff were watching Jason ahead of making a bid.

In June 2010, he signed for Cyrpiot side AEK Larnaca to aid his international career.