[2] He represented South Korea in three editions of the Olympic Games (2000 to 2008), and held multiple national championship titles and swimming records in the relay freestyle and backstroke events (50, 100, and 200 m).

[4] Two years later, Sung won two bronze medals, as a member of the South Korean swimming team, in the men's 400 m freestyle and medley relay at the 2002 Asian Games in Busan, with a total time of 3:23.58 and 3:46.44, respectively.

[5][6][7][8] At the 2005 Summer Universiade in Izmir, Turkey, Sung defeated Japan's Masafumi Yamaguchi and United States' Matt Grevers to earn a silver medal by two hundredths of a second (0.02) in the 50 m backstroke, posting a lifetime best of 25.59.

He broke a new South Korean record on a bodysuit and cleared a FINA B-cut time of 55.43 (100 m backstroke) from the Good Luck Beijing China Open.

[13] Born and raised in Cerritos, California, United States, Sung holds a dual citizenship to train and compete internationally for South Korea in swimming.