Gaggin was born as Mary Eva Gourley on January 9, 1879, in Lawrence, Massachusetts.

Her name was legally changed to Mary Eva Gourley Roe on March 18, 1890, when she was adopted by Syracuse University mathematics professor Edward Drake Roe, Jr.[1][2] She grew up in Cambridge, Massachusetts, and began to write at a young age.

[4][1] She had a child, named John Bridge Gaggin, that died on the day of his birth on July 23, 1919.

[1] In 1939, The Viking Press published her book An Ear for Uncle Emil, which was about a Swiss girl and her doll.

[3] In 1941, Viking Press published her book Down Ryton Water about Pilgrims from the Mayflower ship.