Natsumi Tomonaga was born on August 22, 1991, in Sayama City, Saitama Prefecture, Japan.

[6] Natsumi practiced both swimming and athletics and was a keen athlete, but it was after her breaking the academy's female record for running 1500 meters in 4 minutes and 55 seconds, that Natsumi was scouted for Modern Pentathlon and began training professionally in 2012.

[9][3] In 2016, with only four years of experience, Natsumi represented Japan at the 2016 Rio Olympics and ranked 13th in women's modern pentathlon, the third highest for a Japanese athlete.

[2][8] Shortly after the Olympics, Natsumi's rank was moved up from 13th to 12th due to the disqualification of a top player.

[8] Natsumi continues to compete professionally, finishing 5th in the 2018 Asian Games organized in Jakarta-Palembang[10] and second in the 2019 UIPM 2019 Asia/Oceania Championships.