Deen-Conteh was born in Sierra Leone but grew up in Deptford, England, joining Chelsea at the age of 13. He was a member of their UEFA Champion's League squad for the 2012–13 season, though he did not make an appearance for them. After being released by the club in June 2013, Deen-Conteh went on trial with Millwall in July 2013.
He then signed for Greek club Ergotelis, where he made his professional debut.
Deen-Conteh represented England at youth international level, appearing for the U16 and U19 teams. In March 2013 he committed his international football to Sierra Leone, the country of his birth, after being called up by their senior national team; however, he was unable to play due to an expired passport. He was called up again in May 2013, alongside fellow Chelsea player Samuel Bangura.