Stelios Demetriou won the first trophy in his career with Ermis Aradippou in the Cypriot Super Cup in 2014 against APOEL.
Demetriou signed for Scottish Championship side St Mirren in January 2017. After impressing in his short time at the club, he signed a new one-year deal in June 2017. Demetriou helped St Mirren win the 2017–18 Scottish Championship, but was released at the end of his contract.
On 25 June 2018 Demetriou signed for Scottish Championship side Ross County. He then moved to Macclesfield Town in January 2019.
On 20 May 2018 Demetriou made his debut for Cyprus in a 3–0 friendly match defeat by Jordan. Demetriou came on as an 87th minute substitute, and was booked the following minute.