Miyazawa was born in what is now Hanamaki city, Iwate Prefecture as the eldest son of a wealthy pawnbroker. From an early age, he was disturbed by what he perceived to be the social inequity between his well-to-do family and the impoverished farmers in the area from whom his family profited by lending them money.

In 1918, he graduated from Morioka Agriculture and Forestry College. He was a bright student, so his academic advisor wanted him as an assistant professor. However, differences with his father over religion (he converted to the more activist Nichiren sect), and his repugnance for the family pawnshop business (he yielded his inheritance to his younger brother), created much unhappiness in his early life, and in 1921, he departed Hanamaki for Tokyo.

In Tokyo, while staying with a friend, he was introduced to the works of poet Sakutar? Hagiwara, and was encouraged to start writing. After eight months in Tokyo, when he began to write children's stories, he returned to Hanamaki due to the illness and subsequent death of his younger sister.

He found employment as a teacher in agricultural science at Hanamaki Agricultural High School (?????). Saving his meagre salary, he was able to finance the publication of his first collection of children's stories and fairy tales, Ch?mon no ?i Ry?riten (????????, The Restaurant of Many Orders) and a portion of a collection of free-verse poems, Haru to Shura (????, Spring and Asura) in 1924. Although neither was a commercial success, his work came to the attention of poets K?tar? Takamura and Shimpei Kusano, who admired his writing greatly and introduced it to the literary world.

As a teacher, his students viewed him as passionate but rather eccentric, as he insisted that learning came through actual, firsthand experience of things. He often took his students out of the classroom, not only for training, but just for enjoyable walks in the hills and fields. He also had them put on plays they wrote themselves.

From 1926 until his death in 1933, Miyazawa struggled to improve the material and spiritual life of the impoverished peasants of his native Iwate. He introduced new agricultural techniques and new varieties of seeds. He left his position as instructor at Hanamaki Agriculture School in 1926 to establish the Rasu Farmers Association. At the detached house of his family, where he was staying at the time, he gathered a group of youths from nearby farming families and lectured on agronomy. The association also engaged in plays, music, and other cultural activities.

In 1926 he learned Esperanto and tried to translate some of his Japanese poems into the language; the translated pieces were published in 1953, long after his death.

His writings from this period show sensitivity for the land and for the people who work in it. A prolific writer of children's stories, many that appear superficially to be light or humorous, all contain stories intended for moral education of the reader. He wrote some works in prose and some stage plays for his students and left behind a large amount of tanka and free verse, most of which was discovered and published posthumously. His poetry, which has been translated into numerous languages, has a considerable following to this day. A number of his children’s works have been made into animated movies (anime) in Japan.

Miyazawa's works were influenced by contemporary trends of romanticism and the proletarian literature movement but, above all, were influenced by his devotion to the Lotus Sutra in particular.

Miyazawa struggled with pleurisy for many years and was often incapacitated for months at a time. He died in 1933 of pneumonia.

The Miyazawa Kenji Museum was opened in 1982 in his native Hanamaki city, in commemoration of the 50th anniversary of his death. It displays the few manuscripts and artifacts from Miyazawa's life that escaped the destruction of Hanamaki by American bombers in World War II.

In 1996, to mark the 100th anniversary of Miyazawa's birth, the anime Ihatov Gensou: Kenji no Haru (Ihatov Fantasy: Kenji's Spring; North American title: Spring and Chaos) was released as a depiction of Miyazawa's life. As in the Night on the Galactic Railroad anime, the main characters are depicted as cats.

He loved his native province, and Ihatov (or Ihatovo), the name of the fictional location that appeared in his works, was constructed from the name Iwate (Ihate in the older spelling). Among the variation of names, there is Ihatovo, and the addition of final o is supposed to be the noun ending of Esperanto, whose idea of common international language interested him. This interest is paid tribute to in the 1985 anime adaptation of Ginga tetsud? no yoru (Night on the Galactic Railroad), in which all signs in Giovanni and Campanella's world are written in Esperanto, as well as the written language of the "cats".

In addition to the works mentioned above, Miyazawa's major works also include:

• Night on the Galactic Railroad (??????, Gingatetsud? no Yoru)
• ????? Kaze no Matasabur?
• Gauche the Cellist (Cello Hiki no Goshu)
• The Night of Taneyamagahara (Taneyamagahara no Yoru)
• Vegetarian Taisai

and the poem defining the Japanese ideal,

• Ame ni mo Makezu (Not losing to the rain)