Lee Jang-woo made his acting debut in the 2006 MBC sitcom Hello Franceska. In 2009, he formed the K-Pop project group 24/7 together with Hyun-woo and No Min-woo. They released the single 24 Hours a Day, 7 Days a Week, but disbanded shortly after. In 2011, he was named Best New Actor at the KBS Drama Awards for his work in the daily drama Smile, Dong-hae and the baseball-melodrama series Man of Honor. That same year, he was paired with T-ara’s Eunjung in the MBC variety show We Got Married, which widened his popularity. In 2012, he bagged the Best New Actor trophy for I Do, I Do and Oh Ja-ryong Is Coming. In 2014, he and Han Sun-hwa led the MBC weekend drama Rosy Lovers, which earned him an Excellence Award at the MBC Drama Awards. In 2018, he picked up an Excellence Award at the KBS Drama Awards for My Only Ally. In 2020, he was paired with Jin Ki-joo in the KBS2 series Oh! Samkwang Villa, which won him another Excellence Award at the KBS Drama Awards.

Lee Jang-woo enlisted as an active-duty soldier on April 26, 2016 for his mandatory military service. He was discharged in January 2018.