Hyoyeon was born in Incheon, South Korea on September 22, 1989. She grew up with her parents and her younger brother, Kim Min-gu. She auditioned for S.M. Entertainment at the age of 11 through SM 2000 Casting System. Hyoyeon said she auditioned because her mother brought her to the offices of S.M. Entertainment in the hopes of meeting H.O.T. Hyoyeon then danced in an audition show. In 2004, along with Super Junior's Siwon, she was sent to study Chinese in Beijing. Hyoyeon was also picked as Korea's No. 1 idol dancer on Star News.
Hyoyeon's formal dance training started in elementary school. At her neighborhood's small hip-hop school, she learned hip hop, jazz and Latin dance. In 1999, she enrolled in Winners Dance School, a famous dance school in South Korea specializing in popping, locking, animation and various other hip-hop styles. At the school she met Miss A's Min, with whom she formed the dance team Little Winners (리틀위너스). The duo performed at various showcases and was spotlighted by HipHoper.com in 2004.
On March 7, 2010, she played a cameo role on SBSOh! My Lady starring Choi Siwon and Chae Rim with fellow members Jessica and Sooyoung. On October 17, 2011, it was confirmed that Hyoyeon would be joining the second season of Invincible Youth, a Korean reality show, alongside Sunny. She also gained nicknames such as Hyoding (Kid Hyoyeon), Yoghurt Ajumma, Sancho, Breaky, and Breakomaneci through this show. Hyoyeon joined the second season of Dancing with the Stars, which began airing April 27, 2012. Hyoyeon and her partner, Kim Hyung Seok, ended up winning second place. In October 2012, Hyoyeon took part as a member of SM Entertainment's Younique dance unit which has partnered with Hyundai to release the single "Maxstep". In January 2013, Hyoyeon make an appearance as a guest on Blind Test Show 180 Degrees where she took on the challenge of dancing to various types of club music. In May 2013, Hyoyeon was chosen as an Asian ambassador for the Hong Kong branch of Topshop. On June 17, 2013, it was revealed that Yuri together with Hyoyeon and others would appear as coaches on Dancing 9. She also performed a special collaboration stage with label mate EXO's Kai and Lay on Dancing 9. In June 2014, she make an appearance as a guest alongside fellow member Tiffany and label mate Super Junior's Shindong in Jessica & Krystal show, where the Jung sisters invite them in their 'wannabe house'. On February 12, 2015, she featured in label-mate Amber f(x)'s Shake That Brass music video, in which she plays a trumpet. On March 14, 2015, Hyoyeon featured as a rapper when she did collaboration with boy band S on Immortal Songs 2 which they won with 423 points. On May 6, 2015, it was revealed that Hyoyeon will star in her own reality show on OnStyle called Hyoyeon's One Million Likes. The first episode premiered on June 11, 2015. On June 24, 2015, it was announced that Hyoyeon will be the first Girls' Generation member that will publish a book, titled Hyo Style where she shares tips on beauty, fashion and life. It was published on July 3, 2015. It contains 142 pages and a DVD. On September 10, 2015, it was announced that Hyoyeon will be one of the contestants in a show called Mash Up alongside Seo In Young, MBLAQ's Mir, AOA's Hyejeong and Untouchable's Sleepy. She will be competing with other contestant in DJ-ing. On September 11, 2015, Hyoyeon featured in rapper Vasco's Whoa Ha! music video. She and other rappers lip-sync along in the music video.
On April 3, 2014, it was announced that Hyoyeon had been dating writer Kim Jun Hyung, age 35, for two years. SM Entertainment confirmed that the two had been a couple, but broke up on April 1 following a controversial incident where she accidentally hit him and was reported to the police for assaulting him.
Hyoyeon is close with bandmate Seohyun, labelmate f(x)'s Amber and Miss A's Min. Shinhwa's Min Woo dubbed her as the next BoA and labelmate Super Junior's Eunhyuk also acknowledged her dancing skills.