Drafted by Pantech and Curitel in 2007 and traded off to KTF MagicNs shortly after being drafted, Flash came on to the scene as one of the youngest progamers to be televised at 14 years old. Advancing to the Daum OSL from his first attempt in the offline preliminaries, his career started off in a strong fashion, quickly amassing a winning record in Proleague play and succeeding in getting into both the MSL and OSL. His early notable matches include ones against BoxeR, Bisu, and SaviOr. He became the youngest player to reach the semifinal of the OSL, losing to GGPlay in a series that went to game five. This series is notable for Flash attempting not one, but two 14 CC builds, both on Fantasy in Game One and Five. He subsequently lost both games. Although he put forth strong results, Flash was not a popular player. He was hated for using cheese to eliminate Bisu from the Daum OSL on Monty Hall, and was considered to be a very boring player. It wasn't until later in his career when his genius in relation to economic timings, defensive play, was revealed.

In 2008, Flash's career reached new heights when he defeated Stork in the finals of the 2008 Bacchus OSL and became the youngest Starleague champion ever at 15, defeating top players such as Jaedong and Bisu along the way. It was during this period that he earned his reputation as a TvP master, using his Dual Armory style Flash Build to counter the rampant use of 2 Base Carrier strategies. He also placed first in the GOM Star Invitational, again defeating Stork in the finals as well as Jaedong in the ro8. This was a dramatic turning point in Flash's career as he had previously been criticized for his inability to defend carrier-based builds (shown by his 3rd/4th place match against Stork in the Daum OSL). In the three months after the title win, Flash went on to have one of the most dominant winning sprees in progaming history: achieving a record of 24 wins and 4 losses and tying a Proleague record of 17 straight wins.

Flash stagnated in the year following his OSL victory. While he maintained one of the best Proleague records and consistently beat lesser opponents, he faltered repeatedly in the individual leagues against the very best players. He was knocked out of the GOMTV Classic in seasons 1 and 2 by Jaedong and Bisu respectively, defeated by the 2008 Arena MSL champion fOrGG in the semifinals, and lost in the Avalon MSL to the eventual silver-medalist Kwanro. Though Flash was still known as one of the best Terrans mechanically at this time, his strategies were in question; late turrets against the newer 2 hatchery styles cost him in several leagues, his overuse of 14cc was well known, and Protoss players adapted to his double armory build by rapidly out-expanding him. Nevertheless, he did take gold in Season 3 of the GOM Classic in this period, albeit going through a relatively weaker bracket.

Flash showed remarkable innovations in all his matchups when the 09-10 season started. When KT failed to qualify for the 08-09 Proleague playoffs, Flash had extra time to reflect on his gameplay. In TvZ, Flash displayed a new pre-lair bionic pressure strategy where he used his initial medic marines to delay Mutalisk harassment and then to deny their third gas. Flash also deviated from the infantry based TvZ lategame; whereas Terrans were often having trouble versus Defiler and Ultralisks in the lategame, Flash fortified his expansions with mass tanks and ended up starving out Zergs. In TvP, Flash also showcased use of fac-cc-fac and 4 factory builds, as well as the mass factory build of iloveoov, all surprising departures from his signature late game double armory build. In TvT, Flash displayed remarkable resilience and went on the longest recorded TvT winning streak.

All these innovations led to Flash dominating the 09-10 season. The season was also notable for the numerous "LeeSsang" games that took place between top players Flash and Jaedong (the name meaning "Two Lees" since both have the first name Lee), a rivalry which Flash came out ahead in. Flash took the 2009 EVER OSL against Movie 3-1, notably defeating Jaedong 2-0 in the ro8. Flash then lost the 2009 NATE MSL against Jaedong 1-3, which contained some excellent play from both sides but was probably more remembered for the power outage in game three where Jaedong was awarded the win based on his advantageous position. Flash then went on to essentially win the Winner's League for KT, going 22-3 and crushing Sea, Tyson, and Jaehoon in the finals. Flash again advanced to both individual league finals in the spring. In the 2010 Korean Air OSL, he was defeated by the underdog Effort 2-3 after being up 2-0, where he displayed weaknesses against Effort's early aggression. However, Flash seemed unaffected when several days later he won the Hana Daetoo MSL against Jaedong 3-0, opening 14cc every game. During May 2010, Flash showed some weakness in Ace matches, going 0-5 in them during round 5 of the SPL. However, over the entire year Flash accumulated the most wins in Proleague and helped KT maintain first in the team rankings; Flash also won the final set in the Proleague finals against Hyuk to give the team its first Proleague gold. Flash yet again fought his way into both the Korean Air OSL Season 2 and the Bigfile MSL finals, both against his rival Jaedong. In the MSL, Flash took a close 3-2 victory using several unorthodox timing attacks. For the OSL, the two clashed in Shanghai, China. Jaedong used a burrowed Zergling trap which Flash didn't fall for in game 1, and then surprisingly attempted two four pools in a row, the first of which succeeded. In game 4, Jaedong tried an unorthodox lurker drop which Flash defended to take the series 3-1. Flash thus became the second player after Nada to claim dual individual league golds, and the first player to be in 6 consecutive MSL and OSL finals. He had also defeated his biggest rival Jaedong in 3 consecutive finals. Although Flash placed second in WCG Korea 2010 in which Jaedong took first, Flash was the victor in the WCG 2010 Grand Finals where he defeated both Kal and Jaedong.

In the 10-11 season, Flash has not enjoyed the same degree of success. Although he continues to dominate Proleague, he had two surprising setbacks when he was eliminated from both the 2011 PDPop MSL by rookies Ssak and Classic, and from the 2010 Bacchus OSL in the group stage by Kal and Hyuk. In January interviews, Flash admitted he had been suffering from RSI in his wrist but was optimistic about recovery. However, Flash has since regained his aura of dominance by helping carry his team through SWL 2011, and most recently, winning the 2011 ABC Mart MSL. His road to the championship included going through the Group of Death with Jaedong, Bisu and Sea, trouncing defending champion Hydra in the Ro4 to reach the final where Flash defeated Zero to earn his 3rd MSL title. With this victory Flash has tied NaDa in terms of league titles, being the 2nd progamer in history to have six titles and to possess both the Golden Mouse and Golden Badge. He also led KT Rolster in the SPL playoffs, starting at the bottom of the playoff ladder. After defeating Woongjin Stars, Samsung Khan, and CJ Entus respectively, KT Rolster faced SK Telecom T1 in the finals for the second year in a row. After starting off the series down 1-3, Flash defeated BeSt on Fortress SE to bring the series to 2-3. After Stats beat n.Die_soO on Empire of the Sun, Flash played BeSt in the ace match on Circuit Breakers. After a tense 25 minutes, Flash emerged victorious to give KT Rolster their second SPL Championship in a row.

For the first half of the 2011-2012 season, FlaSh continued to show his dominance in the SPL. However, after reaching yet another finals appearance, once again facing SK Telecom T1, Flash could not pull through. He first lost to Fantasy on Ground Zero, then lost to Bisu in the ace match on Outlier. For the second half of the 2011-2012 season, Flash is currently playing in the 2012 Tving OSL. He advanced from the Dual Tournament to qualify for the Round of 16, where he moved on from a bottom-rung tiebreaker against his teammate Action, and Bogus. In the Round of 8, despite a regame in game 2 where he was in an advantageous position (he won the regame), he convincingly won against Shine 3-1. The semifinals was against the runner-up of the Jin air OSL, Fantasy. Hyped up as the clash of the titans and the last OSL Broodwar TvT, the game attracted quite a bit of attention. However, the results were lopsided, where Flash was swept 0-3, ending his run.