Dempsey booked his first international success at the 1998 Mistral Youth World Championships, where he won the
bronze medal. As a senior his first win was clinched at the North Sea Cup Series in
Pevensey Bay, and an additional third place in the
Kiel Week. During the
2000 Summer Olympics he finished in 16th position. He continued winning competitions in 2001 and beyond like Athens Eurolymp Week, Kieler Woche and the
Spa Regatta before finishing third at the
2004 Summer Olympics to take the windsurfing bronze medal. When switching to the RS:X class he won race after race, resulting in the first spot on the RS:X world rankings. He took the
gold medal at the 2006 RS:X European Championships. At the
2006 RS:X World Championships he finished in 5th position. At the
2008 Summer Olympics he came 4th in the RS:X Class windsurfing class with 60 points. He was 2 points behind Israeli bronze medalist
Shahar Tzuberi. At the
2012 Summer Olympics, Dempsey won the
silver medal in the RS:X Class windsurfing class, behind
Dorian van Rijsselberge of the Netherlands. In the
2016 Summer Olympics, Dempsey again won the silver in the RS:X Class, again behind van Rijsselberge. With three medals won in the Olympics, he became the most decorated men's Olympic windsurfer. In 2017, Nick was awarded an Honorary Doctorate of Science by the
University of Chichester. ==Personal life==