Kim attended Shinil High School in Seoul, South Korea. In September 2005, he competed for the South Korea national junior team, that was runner-up at the 6th Asian Junior Baseball Championship in Seoul, South Korea. At the championship, he went 1-for-4 as a backup outfielder in the South Korean roster.

In November 2005, Kim won the Lee Yeong-min Award, given annually to the most outstanding high school hitter in South Korea.

In the 2007 season, Kim took over the starting job after starting outfielder Yoo Jae-woong was sidelined with a sprained ankle in the beginning of the season. Kim hit .273 with 5 home runs and 32 runs batted in in 99 games, and finished runner-up for Rookie of the Year.

In the 2008 KBO season, Kim won the batting title with an average of .357 and was first in hits with 168. He finished runner-up in MVP voting and won the Golden Glove Award as an outfielder.

In August 2008, Kim competed for the South Korea national baseball team in the 2008 Olympic Games, where they won the gold medal in the baseball tournament. In the preliminary round game against Japan, Kim hit a go-ahead RBI single in the top of the ninth off closer Iwase Hitoki to break a 2–2 tie as a pinch hitter, leading South Korea to a 5–3 win over Japan.

In the 2009 KBO season, Kim exploded offensively, batting .357 and smacking 23 home runs. He defended his hit title with 172, and was runner-up in RBI (104) and triples (6), third in batting average, runs (97), doubles (31), on-base percentage (.448) and slugging percentage (.589), and eleventh in home runs.

On December 11, 2009, he won the KBO Golden Glove Award for outfielder for two consecutive years.

In March 2009, Kim competed for the South Korea national baseball team in the 2009 World Baseball Classic, where they finished runner-up with a 6–3 record. In the tournament, he went 11-for-28 (.393) with 4 RBIs, 6 runs and 7 walks, and was named to the All-Star team at designated hitter.

On December 23, 2015, Kim signed a two-year, $7 million contract with the Baltimore Orioles.

After struggling in spring training, Kim ultimately made the team after refusing a minor league assignment which led to him being booed on Opening Day. On April 10, 2016, Kim played his first career MLB game versus the Tampa Bay Rays. In that game he had two hits and scored a run. Kim finished his first month of Major League baseball having played in only six games, but making an impact, going 9-for-15 (.600), driving in one run, scoring two runs, and having an on-base percentage of .647. He hit his First Major League Career home run on May 29, against the Cleveland Indians that broke a 4–4 tie in the 7th inning.

On July 28, 2017, the Orioles traded Kim and Garrett Cleavinger to the Philadelphia Phillies for Jeremy Hellickson.

On December 19, 2017, Kim signed a four-year, $10.7 million contract with the LG Twins.

In 2018, he represented South Korea at the 2018 Asian Games.