Seohyun was born Seo Ju-hyun on June 28, 1991, in Seoul, South Korea, and is the only child in her family. Her mother was a chairman at a piano school and Seohyun was taught how to play piano, as well as violin and traditional Korean drums in her early childhood. She also learned horseback riding and skating as a child. Her parents did not encourage her to become a Celebrity, but Seohyun credited them as the ones who helped her choose her career path by letting her experience different things. As a fifth grader, Seohyun was discovered by a talent scout while riding in a subway train. She sang children's songs for her audition at S.M. Entertainment, and was accepted as a trainee. Seohyun cited BoA, S.E.S and Fin.K.L. as her main influences to become a singer, expressing "I thought it was cool to show and communicate the emotions through songs". In August 2007, Seohyun made her official debut as the youngest member of girl group Girls' Generation. The group gained significant popularity after the release of their hit single "Gee" in 2009.

Seohyun's early solo work mostly involved songs recorded for side projects and original soundtracks. Although they did not achieve much commercial success; her duet titled "JjaLaJaJJa" with veteran singer Joo Hyun-mi earned a "Trot Music of the Year" nomination at the 2009 Mnet Asian Music Awards. In 2010-2011, Seohyun appeared in MBC's variety show We Got Married 2, alongside CNBLUE's Yonghwa. They played a make-believe couple, portraying what life would be like if they were married. In addition, Seohyun was cast as voice actor for the Korean-dubbed version of animated film Despicable Me (2010) and its sequel Despicable Me 2 (2013), together with Taeyeon. She voiced the character Edith, who is known for her rebellious attitude.

Seohyun graduated from Jeonju Arts High School in February 2010. Until 2011, Seohyun named former United Nations Secretary-General Ban Ki-moon as her life mentor and role model, stating that his book, at one point in time, had helped her endure hard times. In 2013, as an effort to help less-fortunate students with their studies, she donated 100 million Korean won to Dongguk University. She went on to major in theatre acting and graduated from Dongguk University in August 2014, receiving an achievement award at the graduation ceremony. Her fellow Girls' Generation member, Yoona, attended the same university.

In April 2012, a Girls' Generation subgroup named TTS was formed with Seohyun and fellow members Taeyeon and Tiffany. Their debut EP, Twinkle, met with much success and became the 8th highest selling album of the year in South Korea. The subgroup went on to release two more EPs: Holler (2014) and Dear Santa (2015). Besides contributing her vocals to Girls' Generation and TTS songs, Seohyun began to become more active in lyrics writing. She co-wrote the songs "Baby Maybe" and "XYZ" (I Got a Boy, 2013) and was credited as the sole lyricist for "Only U" (Holler, 2014) and "Dear Santa" (Dear Santa, 2015).

From February–June 2016, Seohyun was cast in the Korean version of the musical Mamma Mia. She played a character named Sophie, a 20-year-old bride-to-be, who hopes to find out who her biological Father is on her wedding day. Kim Geum-yeong from CNB Journal commented on Seohyun's improvement from her prior musical performances, praising her "natural acting" and "daring side" while Park Jeong-hwan from News1 claimed Seohyun's performance made the musical "all the more entertaining". Seohyun next played supporting roles in Chinese romantic film So I Married An Anti-fan and SBS's historical drama Moon Lovers: Scarlet Heart Ryeo; the latter earned her a Special Acting Award at the 2016 SBS Drama Awards.

Seohyun, self-described as someone who likes both singing and acting, began to develop an interest in musical theatre. She named musical Actress Ock Joo-hyun, who later became her mentor, as her main influence. Although she had wanted to be in a musical for a long time, she rejected casting offers because she felt unprepared. In January 2014, her wish was fulfilled as she accepted her first musical acting role. Seohyun made her theatrical debut in the musical, Moon Embracing the Sun, which was adapted from the novel of the same name. The musical is set against the backdrop of a Korean traditional palace. Seohyun played the lead role, Yeon-woo, the daughter of a noble family who has a love relationship with both the King and his brother. The musical had previously earned nine nominations at the 2013 Korea Musical Awards.

In January 2015, Seohyun was cast in the role of Scarlett O'Hara in the Korean version of Autant en emporte le vent, a French musical based on Gone with the Wind. She received encouraging review for her performance. Seon Mi-gyeong of OSEN said that while Seohyun's performance cannot surpass that of Bada, who shared the same role, her "fierce voice" complimented and successfully expressed an arrogant Scarlett O'Hara. Kim Hyun-joong of Sports Chosun noted Seohyun's occasional "clumsy" acting, but compared her to Ock Joo-hyun during her rookie days and said that "her ability to bring emotion through song, the most important part of a musical, rivaled that of a veteran actor. Despite a short career, she endlessly showed us various colours and peculiar charms of a woman's life."

In October 2017, Seohyun left S.M. Entertainment after her contract ended. Her Future activities with Girls' Generation remain in discussion.