Kim Seung-soo started acting in 1997 and has since appeared in numerous TV series and films. Some of his early acting appearances include the MBC sageuk Heo Jun in 1999 and the SBS romance series Rookie in 2000. He then became part of the MBC dramas Fox and Cotton Candy and Remember, which both won him Best New Actor at the 2002 MBC Drama Awards. In 2003, he received an Excellence Award at the KBS Drama Awards for One Million Roses. In 2006, he joined the main cast of the MBC sageuk blockbuster Jumong, which scored him another Excellence Award at the MBC Drama Awards. In 2010, he worked with Kim Jung-eun and Lee Jun-hyuk in the SBS series I Am Legend. In 2016, he played the King in the hit KBS2 sageuk Moonlight Drawn By Clouds. In 2017, he starred in the KBS2 daily drama First Love Again, which earned him an Excellence Award at the KBS Drama Awards. In 2022, he joined the main cast of the KBS2 weekend family series Three Brave Siblings.

Kim Seung-soo studied physical education at Kyonggi University.