In November 2003, Jeong competed for the South Korean national team as an amateur player again in the 2003 Baseball World Cup. In the tournament, he batted .308 with 2 home runs and 10 RBIs, leading the team in RBI. In Team Korea's second game against Russia, he went 5-for-6 with a home run and 5 RBIs to lead his team to a 26-3 victory.
Drafted by the SK Wyverns in the second round (15th overall) of the 2005 KBO Draft, Jeong made his pro league debut on April 2, 2005. He had a disappointing rookie season, managing only a .193 batting average.
However, the next season he batted .284 with 45 stolen bases (2nd in the league) and 122 hits, and won the Golden Glove award at second base. After the 2006 KBO season, Jeong was selected for the South Korea national team, and won a bronze medal at the 2006 Asian Games in Doha, Qatar.
In the 2008 KBO season, Jeong hit .300-plus once again (.317), and ranked 2nd in hits (154) and 3rd in stolen bases (40).
On July 16, 2008, Jeong was selected for South Korea national team in the 2008 Olympics. In Beijing, he batted 9-for-29 with 4 runs and a RBI, playing as a utility infielder. In the team's third game of round-robin play against Canada, he smacked a solo home run off Mike Johnson in the third inning that held up for a 1-0 win for South Korea.
On December 11, 2009, he obtained his first Golden Glove Award in the second baseman nomination.