Mary Eloise Donnell was born on November 3, 1865 in Covington, Kentucky. She attended college in Illinois and taught school in Missouri before marrying Frank de Garmo in 1890. During the early part of the 20th century, de Garmo was active in a number of women's organizations. She worked tirelessly to improve 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 several 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. Mrs. 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.

Mary and Frank de Garmo had two children: Margaret de Garmo Payne and Mary Cora Bryan. Mary de Garmo passed away on September 24, 1953, in Chicago, Illinois.