Koyano was born as Misao Nakahiro in Japan on 3 October 1911. Her mother, Kameno, was born around 1889. She married Naohide Koyano and had five children: Henry, Arthur, Chizuko, Joyce, and Victor; only the latter of whom outlived her.
When Koyano and her husband emigrated to the United States, they anglicised their names to Henry and Mary. They owned and worked in a Seattle restaurant, and Koyano was later employed in the cafeteria at Seattle University until her retirement in the 1980s.
Koyano was a keen gardener and prolific poet, winning numerous awards for her poetry. She was reported as being devoted to her family, and her favourite activities were eating candy, watching sports and singing the U. S. national anthem.
Koyano died on 4 October 2021, one day after her 110th birthday. She was survived by her son Victor, as well as seven grandchildren and eight great-grandchildren.
Koyano was not reported in the media; only an obituary was published after her death.
Koyano is believed to have become the oldest living person in the U. S. state of Washington upon the death of 113-year-old Ruth Apilado on 15 August 2021. Following her own death, the title may have passed to Ruth Fear (1911 – 2022).