After graduating, Courtney worked as a physics teacher. While teaching physics at Camden School for Girls, a comprehensive school, he joined Camden National Union of Teachers. In 1998 he founded the NUT's School Teachers Opposed to Performance Pay (STOPP) campaign, to argue against performance-related pay. From 1990–2010 he was the secretary of Camden NUT. He became a member of the NUT's national executive in 2006.