Seo was born on February 27, 1985 in Dobong District (now Nowon District), Seoul, South Korea. She was scouted by S.M. Entertainment and later debuted as the main vocalist of South Korean girl group, M.I.L.K in 2001, under the label subsidiary, SM Entertainment. However, the group soon fell by the wayside due to fierce competition among manufactured bands, which led to one of the members quitting before they completely disbanded in 2003. After M.I.L.K. was dissolved, Seo attended Dongduk Women's University, where she majored in applied musicology to keep her dream of singing career alive. Seo contributed songs to a few soundtracks and SM Town compilations as a solo Artist.
Supporting roles followed in the period drama Hwang Jini (2006), TV police procedural H.I.T (2007) and the queer film Ashamed (also known as Life Is Peachy, 2011). Seo first drew attention for her subtler performances in her career-making performance in The Duo (2011), as a neighborhood tomboy who later becomes a gisaeng. She played her first villain in Feast of the Gods (2012), as an overly ambitious chef caught in a rivalry. Seo was then cast in leading roles for two historical dramas The King's Daughter, Soo Baek-hyang (2013) and The Three Musketeers (2014).
Seo said the character that most resembled her real-life personality was the foodie freelance Writer in Let's Eat 2 (2015), adding that she "didn't realize how much fun shooting a lively and comical drama could be." The drama was Seo's first attempt at romantic-comedy genre, in which she got good reviews and it became her career's turning point.
Her popularity grew rapidly after the hit drama Another Oh Hae-young (2016), which she starred alongside Eric Mun. She was praised for her portrayal of such an ordinary and relatable character, and won the Baeksang Award for Best Actress. Later in the same year, she starred in SBS hit medical drama Dr. Romantic alongside Han Suk-kyu and Yoo Yeon-seok.
In 2017, Seo was cast in the television series Temperature of Love penned by Ha Myung-hee.