Jonathan's older brother Alistair Brownlee won the gold medal in the 2012 Olympic triathlon.
Jonathan Brownlee was the British youth champion in both triathlon and duathlon in both 2006 and 2007. In 2008 he won bronze medals at both the ETU European Junior Triathlon Championships and ITU World Junior Championships, and placed 17th in his ITU World Championship Series début event in Kitzbühel. During the 2009 season he also won gold in the triathlon at the Australian Youth Olympic Festival and competed in two races in the 2009 ITU World Championship Series, finishing 13th in Kitzbühel and 27th in London.
In 2010, Brownlee also represented the French club EC Sartrouville and took part in the prestigious French Club Championship Series Lyonnaise des Eaux, also called Grand Prix. In Dunkirk, at the first triathlon of this circuit, Jonathan Brownlee won the gold medal in the individual ranking and thanks to him EC Sartrouville could win the silver medal.
Despite a 15-second penalty, he won the bronze medal in the Triathon event at the 2012 London Olympic Games. In the five years from 2006 to 2010, Brownlee took part in 22 ITU competitions and achieved 14 top ten positions, among which 5 gold medals.