Koyo Hirai was born in Hakodate, Hokkaido Prefecture, Japan on 10 March 1909. Her family moved to Sakhalin, Russia when she was 9 years old, so she lived with her great-grandmother in Japan. She worked as a housekeeper when she was young. At the age of 19, she got married and eventually had eight children (four sons and four daughters). She was a housewife, devoting her time to take care of her children.

Until the age of 70, Hirai lived in Mikasa, Hokkaido with her husband. Later, she lived with her eldest son and his wife in Sapporo before moving into a nursing home in August 2021. She was always in good physical condition and had no major illnesses. At the age of 103, she was hospitalized and had a surgery after injuring herself in the kitchen, which resulted in a broken femur. She was discharged after about three weeks and was able to walk again after rehabilitation. Until she was around 105, she was physically independent. It was reported she used to drink three bottles of energy drinks every day.

As of September 2021, she had 18 grandchildren, 23 great-grandchildren, and 11 great-great-grandchildren. At the time, her eldest son was 92 years old, and her fourth daughter was 68 years old.

Hirai died in Sapporo, Hokkaido Prefecture, Japan on 8 October 2021 at the age of 112 years, 212 days.