Born and raised in California, Kim enlisted in the United States Navy in the early 2000s before earning a Silver Star and his commission. While a US sailor, Kim also received his Bachelor of Arts (summa cum laude) in mathematics, his Doctor of Medicine, and an acceptance to NASA Astronaut Group 22 in 2017. He completed his astronaut training in 2020 and was awaiting a flight assignment with the Artemis program as of December 2020.
While studying at Harvard Medical School, Kim met and was inspired by astronaut–physician Scott E. Parazynski to apply for Astronaut Candidacy. On June 7, 2017, Kim was one of twelve candidates chosen from a pool of over 18,300 applicants to join NASA Astronaut Group 22. He reported for duty on 21 August 2017, and graduated from training on 10 January 2020. According to NASA, Kim will work in the Astronaut Office while awaiting a flight assignment. On 9 December 2020, NASA formally announced that Kim would join 17 other astronauts in training for a 2024 Moon landing.