The King's Affection is based on Lee So-young's manhwa Yeonmo, and became the first South Korean television series to win an International Emmy Award.

The original manhwa hypothesized a fictional king reigning between the historical kings Yejong and Seongjong, but the production decided to remove any reference to real historical figures, setting the series during the reign of a wholly fictional king and changing the background of the male lead.

Park Eun-bin was reported as the female lead in December 2020, though her agency initially said she was still considering the offer; she took the role partly because she doubted she would ever get another chance to play a king.

Rowoon was reported as the male lead in January 2021, but his agency stated at the time that nothing had been decided as he was focused on filming another drama; both he and Park Eun-bin were officially confirmed on March 19, 2021.

Nam Yoon-su joined the cast because he found the character of Lee Hyun unpredictable and wanted a new experience, while Rowoon was drawn to the love story and wanted to try a historical drama for the first time.

Park Eun-bin took riding lessons and worked on adjusting her voice tone to convincingly portray a woman disguised as a crown prince, aiming to show Hwi as a person facing doubts rather than simply a costume disguise.

Filming began in April 2021 and ended on November 15; production was halted on July 26, 2021 after a crew member tested positive for COVID-19, prompting cast and staff testing and quarantine before filming resumed on July 28.

On August 6, 2021 it was revealed that a day earlier a fire had broken out at the Korean Folk Village in Yongin while the crew was filming there, forcing the cancellation of that day's shoot; firefighters extinguished the blaze within 20 minutes.

The series was shot at numerous real Korean historical locations, including Hangae Village and Hanjujeongsa Temple in Seongju County, Gyeonggijeon Temple and its hanok village in Jeonju, Mungyeong Saejae provincial park, Gwanghalluwon garden in Namwon, and the fortresses Jukjusanseong and Sangdangsanseong.

A broadcast and editing error occurred during episode 15, in which the same scene was accidentally replayed for a few seconds ten minutes into the episode.

Within two weeks of release, the series entered Netflix's global top 10 most-watched programs and reached the top 5 in several Asian countries including Japan, Thailand, and Vietnam; it also placed sixth on NME's list of the best Korean dramas of 2021.