Niimi was born in Yanabe, in the city of Handa, Aichi prefecture, on July 30, 1913. He lost his mother when he was four years old. His literary skill was noticeable at an early age. During his elementary school graduation ceremony, he presented a haiku that impressed most people at the ceremony. At age 18, Niimi moved to Tokyo to enter the Tokyo University of Foreign Studies. He fell sick with tuberculosis while in Tokyo shortly after graduating, and returned to his hometown. He worked there, first as an elementary school teacher, then as a women's high school teacher. He died at age 29. Although not prolific, he shows great talent in all of his writings. His works are known for their accuracy and lively depictions of humans. He is also often compared to Kenji Miyazawa. There is a Niimi Nankichi Memorial Museum in his birthplace, Handa.