She used the Dewey Decimal System and, along with help from her lifelong companion, Lucy Maynard Salmon, built Vassar's into one of the most impressive collections for a liberal arts college at the time.

"[5] Underhill earned her master's degree from Columbia University in where she studied library science and graduated in 1890.

[8][7] With help from Salmon and with Underhill's work, the Vassar library became "one of the most impressive among liberal arts colleges.

[12] In 1916, she and Salmon were among the women voters who came to vote on a proposition to improve the water main system in Poughkeepsie, New York.

[14][3] Underhill also helped collect and organize materials for a biography on Salmon that Vassar professor, Louis Fargo Brown, intended to write.