Kumagai was born on 12 July 1909 in the village of Ōchi (会地村, Ōchi-mura) (now part of Achi), Shimoina District, Nagano Prefecture, Japan.

[2] In 1936, he bought a Pearlette camera (a Konishiroku derivative of the Vest Pocket Kodak), with a simple meniscus lens, and started to use this to photograph village life.

[3] A book of photographs featuring school life published by Iwanami Shoten in 1955 won a photography prize from Mainichi Shimbun.

His photographs are held in the permanent collection of the Tokyo Metropolitan Museum of Photography,[3] and a volume of the series Nihon no Shashinka is dedicated to his work.

[2] The village of Achi created a gallery called Kumagai Motoichi Shashin Dōgakan to permanently exhibit his work.