McDermott began her life's work on an Indian reservation in the Dakota Territory where she was a teacher as well as a friend and adviser to Sitting Bull.
This meeting began a long friendship which resulted in Mother Gertrude traveling to Sitting Bull's headquarters where she encouraged him to surrender before the coming conflict with the U.S. military.
[3][1] One year later, she left the reservation and traveled to Elkton, South Dakota where, with two other sisters, she founded St. Gertrude's Academy.
She served as the superior for St. Gertrude's which functioned as a convent, a school, an orphanage and a hospital until it burned down in 1894.
In this capacity she supervised the opening of Villa Maria, a home for working girls in 1901, St. Vincent Hospital in 1907, St. Vincent School of Nursing in 1910, St. Monica's home for orphans and unwed mothers in 1914, and the Benedictine Hospital in Sterling, Colorado in 1925.