In 2012, he was cast into SM Entertainment through SM’s Global Audition in Vancouver. On December 16, 2013, he was introduced as a member of the pre-debut trainee team SMROOKIES.
In 2014, Mark appeared on Exo 90:2014, a reality TV Show starring EXO, alongside other members of the SMROOKIES program, where they performed songs and recreated music videos of prominent 90s K-pop idols.
In 2015, Mark appeared on Disney Channel Korea's The Mickey Mouse Club as a Mouseketeer alongside other members of the SMROOKIES program, some of which are currently in NCT Dream. The show aired from July 23 to December 17, 2015, and was hosted by Leeteuk of Super Junior.
2016–present: NCT, solo endeavors
On April 9, 2016, he officially debuted as a member of NCT, in the unit NCT U, with the single "The 7th Sense".
On July 7, 2016, he debuted as a member of the 2nd unit, NCT 127, with their mini album NCT #127.
On August 25, 2016, he debuted as a member of the 3rd unit NCT Dream, with the single "Chewing Gum".
In January 2017, he joined High School Rapper, a survival hip hop reality TV show which aired on Mnet. He made it to the finals and performed an original song called "Drop" featuring labelmate Seulgi of Red Velvet. He subsequently placed 7th overall.