"The King: Monarch of Eternity" is Lee Min-ho's first acting role since he was discharged from the South Korean military on April 25, 2019.
Lee Min-ho and screenwriter Kim Eun-sook previously worked together in 2013 SBS drama series Sangsogjadeul (2013).
Min-ho Lee and Go-eun Kim both played high schoolers in Eun-sook Kim's dramas. Min-ho Lee played a high schooler in Heirs and Go-eun Kim played a high schooler in Goblin.
Actress Kim Go-eun and screenwriter Kim Eun-sook previously worked together in 2016 tvN drama series Goblin (2016).
Director Baek Sang-Hoon and screenwriter Kim Eun-sook previously worked together in 2016 KBS2 drama series I discendenti del sole (2016).