Kim Dong-hyun participated in the second season of the Mnet idol competition Produce 101 in April 2017, where he finished in 28th place. Later that year, he was launched as one half of the musical duo MXM along with his labelmate and fellow Produce 101 contestant Im Young-min. In September 2017, they released their debut extended play titled Unmix. In August 2018, they dropped their first studio album More Than Ever, which featured the lead single Ya Ya Ya. In May 2019, the duo debuted as part of the new boy group AB6IX with the extended play B:Complete. A few months later, their first full-length album called 6ixense hit the market.
Kim Dong-hyun bagged his first acting gig as a male lead in the 2021 web series Convenience Store Fling. He later starred as one of the members of the fictional band LUNA in the SBS musical drama Let Me Be Your Knight.
Kim Dong-hyun has an older fraternal twin brother.