Gu Hye-sun made her television debut in 2004 with appearances in KBS2’s Drama City and MBC’s Nonstop 5. In 2006, she starred in the hit KBS1 daily series Pure in Heart, which won her Best New Actress at the KBS Drama Awards. In 2007, she won the New Star Award at the SBS Drama Awards for her work in the sageuk King and I. In 2009, she gained widespread fame after starring as the female lead in the popular KBS2 series Boys Before Flowers, opposite Lee Min-ho. Her performance in the drama earned her many acting awards, including an Excellence Award at the KBS Drama Awards. In 2014, she acted alongside Lee Sang-yoon in the SBS series Angel Eyes, which scored her an Excellence Award nomination at the SBS Drama Awards.

Aside from acting, Gu Hye-sun also branched out into other artistic fields. In April 2009, she published her debut novel titled Tango, which sold over 30,000 copies in only a week after its release. In July 2009, she held her inaugural solo exhibition in Seoul, which featured around 40 of her illustrations. In October 2009, she dropped her first album called Breath. That same year, she made her directorial debut through the short film The Madonna. It won the Audience Award at the 26th Busan Asian Short Film Festival and the Spotlight Award at the 12th Short Shorts Film Festival & Asia.

In March 2016, the agencies of Gu Hye-sun and her Blood co-star Ahn Jae-hyun confirmed their relationship. They married in May 2016. In 2020, the couple became legally divorced after going through a messy public spat.