Ryu was born in Hahoe Maeul, Andong, Gyeongsang province (today a UNESCO World Heritage Site), to a yangban family of the Munhwa Ryu clan.
Ryu is said to have been so precocious that he absorbed the teachings of Confucius and Mencius at the age of 8. In 1564 the 19th year of Myeongjong, he passed the Samasi examination, and in 1566 he passed the Mun-gwa at a special examination, and then took the post of Gwonji bujeongja (권지부정사, 權知副正字). He held various other positions and in 1569 he joined the imperial birthday mission to Ming as a Seojanggwan (서장관, 書狀官), returning to Korea the following year.
Thereafter he held posts including Inspector of Classics (경연검토관, 經筵檢討官) and devoted himself to editing, being granted a royal sabbatical (사가독서, 賜暇讀書). Subsequently he held posts including Gyori (교리, fifth jeong rank) and Eunggyo (응교, 應敎, fourth jeong rank). He was appointed Jikjehak (직제학, 直提學) in 1575 and Bujehak (부제학, 副提學) in 1576. Continually he held posts including Doseongji (都承旨), Daesaheon (대사헌, 大司憲) and Daejehak (대제학, 大提學).
In 1590, he was appointed Uuijeong (Third State Councillor) and Pungwon Buwongun (풍원부원군, 豊原府院君). In 1591, he was promoted to Jwauijeong (Second State Councillor) and Ijo Panseo (이조판서, Minister of Personnel, the first ranked of the six Ministries). However, the Easterners faction split into the Southerners and the Northerners. Ryu Seong-ryong was a Southerner (claiming exile, instead of death, for Jeong Cheol, the leader of the Westerners rival faction).
He was in the rank of provincial Dochechalsa (도체찰사, 都體察使) when the Imjin War broke out in 1592 also he was appointed Yeonguijeong, the Chief State Councillor. Ryu Seongryong accompanied the royal family from Hanseong to Uiju. In this capacity, he oversaw all military units and called leaders like Yi Sun-sin and Gwon Yul to battle. He also fought on the Korean-Chinese allied forces side in the Siege of Pyongyang. He suggested of establishment the Hunnyeon Dogam (훈련도감, 訓鍊都監, Training capital garrison).
In 1598, he was ousted by the Northerners faction. King Seonjo rehabilitated him. However, he refused to take office as a minister in 1600. In 1602, the Joseon government bestowed honors upon him, as the second rank of Hoseong Gongsin (호성공신, 扈聖功臣), and appointed Pungwon Buwongun again.
After which he spent his time on political writing before his death in 1607.
Ryu's major writings are preserved in the Seoaejip (The Anthology of Seoae, 서애집, 西厓集), Jingbirok (The Book of Corrections, 징비록, 懲毖錄), and minor writings as Hwanghwajip (황화집, 皇華集), Jeongchungrok (정충록, 精忠錄). Ryu Seong-ryong was enshrined in the Byeongsan Seowon and Hogye Seowon in Andong, North Gyeongsang.Portrayed by Lee Jae-ryong in the 2004-2005 KBS1 TV series Immortal Admiral Yi Sun-sin.