He competed in the 2004 Summer Olympics in Athens and finished 6th in the double-handed Star class with Steve Mitchell. The same team received a bronze medal at the 2005 World Championships and a gold medal at the 2005 European Championships.
Percy won World Bronze in the Star Class with Andrew Simpson in 2007 and European Championship Gold in 2009. The duo have just won Gold in the same class of boat at the 2010 World Championships which took place in Brazil. With previous partner, Steve Mitchell, Percy won the 2002 Star World Championships in Los Angeles, and medalled in the next 3 world championships, winning the Bronze medal in each of them.
In 2005, Iain decided to take a break from full-time Olympic Class sailing and join America’s Cup challenger +39 Challenge as helmsman. Due to limited funding, +39 only managed ninth place in the Louis Vuitton Cup. Iain then joined Great Britain’s TEAMORIGIN competing in the Louis Vuitton Trophy regattas (Nice, Auckland, La Maddalena) and on the TP52 circuit, prior to stepping onboard Artemis Racing for the Louis Vuitton Trophy – Dubai.
Percy has been with Artemis Racing since then and is now Sailing Team Director for the Challenger of Record for the 34th America's Cup which will be held in San Francisco, California during the summer of 2013.