Sakai was born in Hirakata, Osaka Prefecture, Japan, on 12 June 1909. She had two children. She lived independently in Osaka until the age of 98, eventually moving to Tottori Prefecture.
At the age of 110, she resided at the Shin Inaba Koho-en Long-Term Care Health Facility (Japanese: いなば幸朋苑) in the district of Hamasaka, Tottori City. In September 2020, it was reported her eldest daughter was 73, and she had five grandchildren and four great-grandchildren at the time. According to her daughter, the secret to her long life was that she ate what she liked (greasy food and often french fries and hamburgers) and didn’t let her stress build up.
Natsuko Sakai died in Tottori City, Tottori Prefecture, Japan, on 7 August 2021 at the age of 112 years, 56 days.
On 26 March 2020, following the death of 111-year-old Toshie Yasuda of Kurayoshi City, she became the oldest living person in Tottori Prefecture, Japan.
On 6 November 2020, at the age of 111 years, 147 days, she surpassed Yasuda’s age and became the oldest person ever in Tottori Prefecture.
Her age was verified by Japan’s MHLW, Ryohei Asano and Yu Li, and validated by LongeviQuest on 13 March 2023.