Sara Mendaro Guichón de Valenzuela was born in the city of Florida, Florida Department, Uruguay on 21 December 1911. She was the youngest of 10 children born to parents Eugenio Mendaro and Gregoria Guichón.
On 18 April 1936, she married Hermecendo Valenzuela. The couple had six children. She was baptized by the Adventist Church on 23 December 1955 at the age of 44. Since her baptism, she had participated in many activities to promote the Church. The couple had a company that exploited construction materials, she did the accounting, paperwork, and customer service. The couple was married for more than 50 years before her husband died. She never smoked or drank alcohol.
From the age of 100, she celebrated her birthdays every half year because she said that you have to live to the fullest. On 31 March 2021, aged 109, she got vaccinated against COVID-19. She lived with her daughter Ruth.
Mendaro Guichón died in Durazno, Uruguay on 4 February 2022 at the age of 110 years, 45 days. At the time of her death, she had 18 grandchildren, 31 great-grandchildren, and one great-great-granddaughter.
Her age was verified by Esteban González Olase, and validated by the LAS on 23 July 2021.
At the time of her death, she was the second-oldest known living woman in Uruguay, behind Blanca Luisa Cuitiño Barbier and the third-oldest known living Uruguayan person overall, behind Cuitiño Barbier and Félix Ortíz Galbán.