Born on August 27, 1896, the eldest son of Miyazawa Masajiro, running the pawnbroker and secondhand-clothes shop 'Miyazawa Shoten' in Satokawaguchi village, Hienuki District, Iwate (today Toyosawa-cho, Hanamaki). His birth came two months after the occurrence of the Meiji Sanriku Earthquake and the Meiji Sanriku great tsunami (June 15) of that same year, and the Rikuu Earthquake also occurred five days later. Throughout his life he had to face natural disasters in Tohoku. After Hanamaki-kawaguchi Ordinary Higher Elementary School and Morioka Middle School, in 1915 he entered Morioka Higher School of Agriculture and Forestry (today Iwate University Faculty of Agriculture) at the top of his class, graduated top in 1918, and remained as a research student for two years. A devout believer in the Lotus Sutra, he ran away from home in 1920 to Tokyo and entered the Kokuchukai (a Nichirenist group headed by Tanaka Chigaku), but in 1921, because of his father's illness, he returned to Hanamaki. In December 1921 he took a post as teacher at Hanamaki Agricultural School (to 1926), becoming a legendary teacher loved by his students. In this period he published 'The Restaurant of Many Orders' (1924, a children's story collection at his own expense) and 'Spring and Asura' (1924, a poetry collection at his own expense), but in life he was almost unknown. In 1926 he resigned from teaching and founded the family farm 'Rasu Chijin Association,' beginning an idealistic movement to teach poor farmers agricultural techniques, fertilizer design, and arts (music and literature) free of charge. But the strain of the activity caused him to develop tuberculosis in 1928. While fighting the illness he wrote children's stories such as 'Night on the Galactic Railroad,' 'Wind Boy Matasaburo,' 'Gauche the Cellist,' and 'The Life of Gusuko Budori,' and poems in 'Spring and Asura, Second Collection, Third Collection,' but many remained unfinished or unpublished. At 1:30 p.m. on September 21, 1933, he died of acute pneumonia at the family home in Hanamaki at 37. There is an anecdote that just before his death he showed his father the notebook with 'Be Not Defeated by the Rain.'

A rare character in which deep religiosity (faith in the Lotus Sutra), scientific spirit, dedicated love for peasants, and artistic sensibility — all were fused. His thought that 'until the whole world has become happy, there can be no happiness of the individual' (from 'Outline of the Essentials of Peasant Art') was a belief that ran through his life. A genius of languages, he mastered Esperanto by self-study, played the cello and organ, and learned English, German, French, and Latin. On the other hand, he was sickly and sensitive, and also had a darkness that constantly faced a foreboding of death. With little experience of love, he stayed single all his life, and his deep spiritual bond with his younger sister Toshi — her death (1922) became the source of poems such as 'The Morning of Eternal Parting.'

Nearly unknown in life, Miyazawa Kenji suddenly appeared in literary history after his death and is loved today as a national writer. In 1934 the sorting and publication of his manuscripts began with Kusano Shinpei and others, and 'Night on the Galactic Railroad,' 'Wind Boy Matasaburo,' and 'The Life of Gusuko Budori' became classics of Japanese children's literature. 'Be Not Defeated by the Rain,' after its discovery in 1934, has been widely recited as a national poem, and was also cited as a symbol of reconstruction after the Great East Japan Earthquake (2011). Hanamaki City, Iwate, has unfurled a regional branding bearing the name 'Ihatov' (the name of a utopia Miyazawa proposed), and the Miyazawa Kenji Memorial Hall, Fairy Tale Village, and Ihatov Hall stand together. 'Night on the Galactic Railroad' has been made into film (the 1985 anime version), musicalized, and has greatly influenced the Ghibli-style world view (implied in 'Spirited Away' and so on), having a great influence on modern pop culture as well. UNESCO has highly rated Miyazawa's view of nature and thought of peace, and international commemorative events were held in 1996 for the centenary of his birth. The Iwate University Faculty of Agriculture has a materials room with Miyazawa's autograph manuscripts and the like.