Demo was born and grew up in Nitra, and he started his professional career playing for the local team FC Nitra. He also played for K Slovan Bratislava before being transferred to the Dutch side PSV Eindhoven in 1998. He moved to Germany to join Borussia M nchengladbach in 1999, and played there for six seasons. In 2005, Demo was signed by the Austrian team Grazer AK, but could not secure a spot on the match-day squad due to injuries, and his contract was terminated with mutual agreement on 5 January 2010. He then returned to his first club, FC Nitra.
Demo has represented his country in 24 international matches, in which he has scored four goals.