After appearing in several dramas in minor or supporting roles, he first came to prominence with domestic audiences in the highly-rated 2003 daily soap Yellow Handkerchief and portrayed lovers with his future wife.

[16] He was cast in Sad Love Story as a last-minute replacement for Song Seung-heon, who was embroiled in a scandal involving his exemption from mandatory military service and forced to pull out.

[17] Yeon himself nearly did not get the role as his character's background story required him to film on location in New York City for several weeks and he had to get a visa at short notice.

Although domestic viewership was lower than expected, Yeon was praised for ably stepping into the role at the last minute and it led to a rise in popularity for him, especially in Japan.

[20][21] His comeback project was the critically-acclaimed 2008 generational drama East of Eden in which he and Song Seung-heon portrayed brothers who grew up in a mining town during the turbulent 1970s and later find themselves on opposing sides of the law.

[24] From 2011 to 2012, Yeon, an avid car enthusiast and former Formula 4 driver, was a co-host of Top Gear Korea and hosted the first three seasons before leaving to concentrate on acting and other interests.

In the 2012 cable drama Can Love Become Money, he played the CEO of a hospitality company who hides his sensitive and caring side behind an unpleasant and emotionless exterior.

[28][29] He reunited with his East of Eden co-star Han Ji-hye the following year in Pots of Gold, in which he portrayed the eldest son of a chaebol family embroiled in a saga involving his father's mistresses.

[57][58] He was previously a United States permanent resident but gave up his green card when he entered the entertainment industry, thus making him eligible for conscription.