Diener won his first international championship title at European Junior Championships 2010 in 50m backstroke. One year later, in 2011, he defended his gold medal and won also in 100m and 200m backstroke at European Junior Championships in Belgrade (Serbia) and won gold medal at World Junior Championships in Lima (Peru) in 50m backstroke. Christian Diener won silver in 50m backstroke at European Short Course Swimming Championships 2013 and 2014 at European Aquatics Championships. He was member of German team at the 2016 Summer Olympics in Rio de Janeiro and competed in the 200m backstroke, finishing 7th.