Jun was born in Seoul, South Korea, and studied in the Department of Theater and Film at Dongguk University. Jun's childhood dream was to be an airhostess, but at the age of 16, a neighbour introduced her to modelling. She began her career as a fashion model for Echo Magazine in 1997.


After appearing in a number of TV sitcoms, her movie debut came in White Valentine (1999). This was followed by a role in Il Mare (2000), which proved to be a success. An American remake of Il Mare, The Lake House, was released 14 June 2006. However, her biggest breakthrough was in My Sassy Girl (2001), a romantic comedy that bought her international recognition, and won her a Best Actress award at the Grand Bell Awards in 2002. It was the highest grossing Korean comedy film at the time. In 2004, she starred in the film Windstruck, a South Korean fantasy-romantic comedy which was directed by Kwak Jae-yong. The film was also another major success and was the 8th-highest grossing Korean film of 2004. In 2006, she was also in the movie Daisy. In 2008, she was also in the film A Man Who Was Superman.

Ji Hyun worked closely with CJ Entertainment, particularly with the distribution of Windstruck, The Uninvited, and A Man Who Was Superman.

In 2009, she made her English-language film debut when she starred as Saya in Blood: The Last Vampire alongside Masiela Lusha. The movie was filmed in China and Argentina in March 2007. It was during filming and before the March 2009 release that she adopted the Westernised name Gianna Jun. The film was Jun's first official action film and was a "drastic" change in her image from previous films.

As of February 2010, she began filming Snow Flower and the Secret Fan in China.