John Korir grew up the seventh of eight children in Cherangany, Trans Nzoia County, in Kenya's Rift Valley. His older brother Wesley had already won the Boston Marathon in 2012 and later served as a member of the Kenyan parliament. Wesley insisted John finish school before pursuing athletics, and John tagged along to training sessions from a young age. Korir's marathon career started quietly. He debuted at the 2018 Ottawa Marathon with a 2:09:14 second-place finish, then won the LA Marathon in both 2021 and 2022. The jump to the elite level came at the 2024 Chicago Marathon, where he ran 2:02:44 for the win, the second-fastest time in the race's history. At the 2025 Boston Marathon, he took a fall near the start line, lost his bib, fought his way back to the lead pack, and still won in 2:04:45. He and Wesley became the first brothers to both win the Boston Marathon. After a DNF at the 2025 Chicago Marathon, Korir bounced back to win the Valencia Marathon in a personal best 2:02:24. Away from racing, the Korir brothers fund the Transcend Talent Academy in Cherangany, providing scholarships to young Kenyan athletes.