Kang Haneul debuted as an actor in the 2006 musical The Celestial Watch. The following year, he landed his first television acting role in the KBS2 series My Mom, Supermom. He then played supporting roles in the SBS dramas To the Beautiful You and Heirs, which aired in 2012 and 2013, respectively. In 2014, he portrayed the teen counterpart of Lee Sang-yoon’s character in the SBS series Angel Eyes, which won him the New Star Award at the SBS Drama Awards. Also in 2014, he became part of the main cast of the hit tvN workplace drama Misaeng. In 2015, he appeared in the coming-of-age movie Twenty, which gave him several Best New Actor nominations and wins. In 2016, he starred in director Lee Jun-ik’s black-and-white biographical period drama film Dongju: The Portrait of a Poet, which bagged the Daesang at the 52nd Baeksang Arts Awards. Later in 2016, he joined Lee Jun-ki and IU in the SBS sageuk Moon Lovers: Scarlet Heart Ryeo, which earned him an Excellence Award at the SBS Drama Awards. In 2019, he acted alongside Gong Hyo-jin in the top-rating KBS2 series When the Camellia Blooms, which took home the Daesang at the 56th Baeksang Arts Awards. His work in the series also scored him numerous accolades, including the Baeksang Best Actor trophy.

Kang Haneul attended Chung-Ang University. He enlisted for his mandatory military service in September 2017 and was discharged in May 2019. He revealed in an episode of the JTBC travel show Traveler 2 that he is claustrophobic.