2012–2018: Career beginnings and rising popularity
At 20 years of age, Jang began his modelling career through the 2012 S/S General Idea Collection Show during Seoul Fashion Week. According to Jang, modelling slowly reversed his timid and introverted personality from childhood. Despite recalling his first year as slow-paced, his credits in fashion shows at the time included Kolon and Caruso, and runway show credits included Loriet (로리엣) and Unbounded Avenue (언바운디드어위). He was also selected as one of twelve promising rookie models by the Korea Model Association. In 2013, his presence in the music videos "The Red Shoes" and "Friday" by IU brought him public popularity. He went on to win the Fashion Model Award at the 2014 Asian Model Award ceremony in January 2014.
Jang was met with an opportunity without the requirement of an audition by the director of It's Okay, That's Love (2014) to appear with label-mate Lee Sung-kyung. His role created a buzz within South Korea despite having no lines and was dubbed the "One Second Cameo". Later that year, taking acting more seriously, Jang auditioned for The Greatest Marriage (2014). While many other actors auditioned for the role, Jang was selected as he matched the role and its image more in comparison to the others. He was also cast in the drama Schoolgirl Detectives (2014). Jang's acting portfolio expanded with his supporting roles in television series This is My Love (2015), Beautiful Mind (2016) and The Liar and His Lover (2017) alongside web-dramas We Broke Up (2015) and Love for a Thousand More (2016), both co-productions of YG Entertainment and starred label-mate Kang Seung-yoon. In 2016, he challenged himself with hip hop after being featured as a contestant in the second installation of Tribe of Hip Hop. His performance to "Doom Dada" by T.O.P of Big Bang garnered attention due to its fast-paced tempo and flow while its execution was praised despite his occupation as a model. Jang finished as a finalist among the top 10.
In 2017, he gained popularity with his supporting character in Confession Couple (2017) as Jang Na-ra's college senior. Jang cited the drama as a turning point in his acting career as he received various offers to appear in commercials and television shows and felt its effects when he was approached or recognized by people in their 40s and 50s. With Yang Se-jong and Woo Do-hwan, Jang was dubbed a part of "Most Popular Actors Born in 1992" and "Rising Super Rookies" by media outlets in South Korea. After its airing, he received casting offers for various dramas including My Mister (2018). Jang chose to cast in the series for its director and writers as well as the character. It also served as an on-screen reunion with IU since 2013. After it aired, Jang gained further recognition with his role as a cold-hearted loan shark. His performance earned him the Best New Actor award at the 6th APAN Star Awards and nominations at The Seoul Awards, marking his first as an actor.
In 2018, Jang was cast alongside Jin Ki Joo in his first leading role in the melodrama Come and Hug Me, taking on the role of a detective who has a fated relationship with his childhood friend. His performance in the drama earned him the Best New Actor award in television at the Baeksang Arts Awards.
