Mary Eloise O’Donnell de Garmo was born on November 3, 1861, in Covington, Kentucky, to Dominick and Sarah Jane Wingler O’Donnell. She taught school before moving into a career of club work throughout the early 20th century. She largely focused on improving children's health care, serving as the national chairperson of the Department of Good Roads for Child Welfare and founding the Better Babies movement. The first Better Babies Contest was held at the Louisiana State Fair in 1908 and national events, held for children aged six months to three years, followed soon after. During the 1930s, de Garmo served as the President of the Louisiana Mothers' Congress and as the President of the National Mothers' Union.

de Garmo was also active in organizations dedicated to preserving the memory of those who had died in past wars. She founded and served as President of the National Society of 1917 World War Registrars and created the Gold Star Tree Court of Honor Society to honor those who had died in World War I. de Garmo also held a position as a Trustee to the National Society of U. S. Daughters of 1812 and was selected as Chief Acting Registrar in the War Department of Missouri.

She married Frank de Garmo (b. 1860) in 1890 in Missouri, and together they had two children: Mary de Garmo Bryan (1891-1986) and Margaret de Garmo Payne (b. 1893). Mary de Garmo died September 24, 1953, in Chicago, Illinois.