Casilda Benegas was born in Trinidad, Itapua, Paraguay on 8 April 1907. Her parents were Patricio Benegas and Asuncion Ojeda. She was baptized in Santisima Trinidad, Trinidad. She grew up in a Guarani-speaking household. She married Benigno Gallego Cuenya in Santisima Trinidad. On 6 April 1945, she moved to Argentina.
In 2000, at the age of 93, she emigrated to Spain, but 13 years later, she returned to Mar del Plata to settle permanently.
On 10 December 2020, Benegas tested positive for COVID-19 during the coronavirus pandemic but was asymptomatic. By 23 December 2020, she had successfully recovered, making her the oldest known survivor of the disease, until Lucile Randon survived the disease in January 2021. However, she was not the oldest known living survivor, as Maria Branyas Morera of Spain, who recovered from COVID-19 in April 2020, is five weeks older.
On 30 March 2021, at the age of 113, Benegas got vaccinated against COVID-19, making her one of the oldest people to receive the vaccine.
Following the death of Antonia da Santa Cruz on 23 January 2022, Benegas became the oldest validated living person in Latin America.
In December 2020, Benegas recovered from COVID-19 at the age of 113 years and 8 months; this was the oldest age at which anyone is confirmed to have contracted the disease and survived, until Lucile Randon survived the disease in January 2021. She was the second-oldest living survivor of COVID-19, behind Maria Branyas Morera who survived the disease in April 2020, shortly after her 113th birthday.
Casilda Benegas died in Mar del Plata, Buenos Aires, Argentina on 28 June 2022, at the age of 115 years, 81 days.