Fay was born about 1804, probably in Vermont. She was the daughter of Ethan Allen Fay and his first wife, Catharine Street Fay. Raised in Albany, New York, she received what contemporaries praised as a fine classical education. Fay eventually made her way to Virginia and by 1839 was working as a governess for the Dulany family at Shuter’s Hill, an estate in Fairfax County. During the next decade she also worked in Loudoun County for another branch of the Dulany family. Fay returned to New York. In 1840 she established and served as principal of the Glen Falls Female Academy, but soon she moved to Essex County, Virginia, where for a few years she was principal of Midway Female Academy. When living in Fairfax County, Fay attended Christ Episcopal Church in Alexandria. In later years she credited Bishop William Meade and the church’s rector, Charles Backus Dana, with having inspired her to enter the mission field.
In 1850 Fay volunteered to serve at the Episcopal mission in China. Departing from New York on November 8, 1850, she arrived in Shanghai, the church’s main station, the following March. Although not the first single woman in the Episcopal mission, which had been established in 1845, Fay became its longest-serving missionary and a key figure in its long-term success. Shortly after her arrival she took charge of the boys’ school that the mission had established in 1846. Becoming proficient in Chinese, Fay supervised students in the translation of classic Chinese texts into English, taught a Bible class, and gave instruction in both English and Chinese composition. In addition to directing the school’s finances, she ensured that her charges were properly fed and clothed, and she also nursed them through frequent bouts of illness. After the outbreak of the(1861–1865), funding for the Episcopal mission collapsed and forced the school to close. Although still counted among the American mission, Fay taught during the war years at the school operated by the Anglicans’ Church Missionary Society. During 1863 and 1864 she traveled in Europe.
After the war, the mission recovered rapidly, thanks in part to greater concessions granted by the Chinese government and to the end of the Taiping Rebellion, and Fay soon reopened the boys’ boarding school. A devoted student of Chinese texts and fully committed to the goal of Christianization, she brooked little frivolity. One observer remarked that her “work seemed all granite—nothing to please or amuse an audience—it seemed a man’s work done by a woman.” Still, Fay’s tireless efforts inspired much devotion from her scholars, the Chinese Christian community, and other members of the Episcopal mission. Her goal of training students to carry forward the work of the mission achieved notable success. Ten of her students went on to take holy orders in the Episcopal Church. Encouraging her Chinese assistants and older students to shoulder more of the academic instruction also freed her for administrative work and allowed her to extend her supervision to several day and boarding schools that the mission operated in Shanghai for boys and for girls.
When not running her schools, Fay remained an active student of Chinese language and culture. During the 1860s and 1870s she published at least eight translations of catechisms and other theological works into Chinese and into the Shanghai dialect. In frequent contributions to Episcopal periodicals, Fay interpreted facets of Chinese culture for her American readers and asserted that educating the youth was the best means of evangelizing the Chinese. In 1872 she published a translation of the elaborate marriage ceremony recently conducted for the emperor. Fay’s most significant scholarly achievement was her assistance in compiling Samuel Wells Williams’s Syllabic Dictionary of the Chinese Language (1874). Williams credited Fay and Tsang Chu-Kwei, her Chinese colleague, with revising approximately 60,000 phrases.
Because of declining health, Fay spent the years 1870 and 1871 in the United States. In November 1876 Fay’s boarding school was expanded to include a theological department and renamed Duane Hall and Divinity School. Fay was honored at the opening ceremonies for her many years of service. Three years later Duane Hall merged with another Episcopal boarding school in Shanghai to form Saint John’s College (later University). Fay continued to teach at Duane Hall until a long period of ill health led to her removal to Yantai (then known by westerners as Che-Foo) in Shandong Province. Fay died there on October 5, 1878, and was interred at a Christian burial site just outside Yantai.