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. He had played 167 times for Orient. In 2013, he moved to AEK Larnaca. On 1st July 2015, he signed for Walsall whose manager Dean Smith was at Orient. "I have known Jason for a long time," said Smith. "He will bring bags of energy, great technique and a winning mentality. He has the know-how and drive that we are looking for. He's an extremely talented player so I'm delighted that we have been able to persuade him to come here."

Demetriou added: "I owe a lot to Dean Smith. I worked closely with him when I was coming up through the youth ranks at Leyton Orient. I've missed the buzz of the English game and this opportunity has come up at the right time for me."

At the time of his return to England, he has won 24 caps for Cyprus. He retired from international football in 2020, having won 51 caps and scoring once.

Demetriou signed for Southend on a two year contract on 23rd June 2016. He played 53 times for Walsall in 2015-16 as they lost in the League One play-off semi-finals, but rejected a new deal.

Southend were relegated to the National League on 1st May 2021

On 14th October 2021 Demetriou took temporary charge of Southend following the sacking of Phil Brown on 9th October 2021. Southend later appointed Kevin Maher as their new manager.

At the end of July 2023 he was added to Maher's coaching team for the 2023/24 season.