[3] In 1881, Phelps married Simon Henry Gage (1851–1944), who was already an assistant professor of histology and embryology at Cornell, where he would spend his entire academic career.

Like many women scientists in the late 19th century, Gage never held a formal job congruent with her abilities and spent some of her time supporting her husband's career—for example as an editor of at least one edition of his book The Microscope and as an illustrator for some of his papers.

When she began her career, the fibrous aspects of striated muscles in small animals were not well understood, and Gage's research in this area came to be considered fundamental.

In 1905, she decided to study neurology formally, first attending Johns Hopkins Medical School in Baltimore, Maryland, and then moving to Harvard University.

These studies influenced the course of her research into the comparative morphology of the brain and its embryological development, as well as into the anatomy of the human nervous system as a whole.

Following Gage's death, her husband and son set up a memorial fund in her name that was used to build a room in the new women's dormitory at Cornell, Clara Dickson Hall.

[5] A set of histological slides of sections of mouse brain prepared by Phelps in 1894 is held by Yale University.