Gill was educated at Smith College. After graduating in 1881 she remained in Northampton, Massachusetts, as a faculty member of Miss Capen's School for girls, operated by her aunt. She taught mathematics for 17 years, taking time off to receive a master's degree from Smith in 1885 and to pursue advanced studies at the Universities of Leipzig (Germany), and Geneva (Switzerland), and at the Sorbonne in Paris during 1890–93. On the outbreak of the Spanish-American War in 1898, she joined the Red Cross, taking charge of the first party of Red Cross nurses sent to Cuba. She later was responsible for nurse selection and placement at army hospitals in Tennessee and on Long Island, New York. After the war she helped organize the schools of Cuba under the governorship of General Leonard Wood, and she also undertook relief work among Cuban orphans. In 1901 she was named dean of Barnard College in New York City. During her seven years in that post she planned and saw begun in 1907 the college's first dormitory and established the Bachelor of Science degree.

In 1909, at the request of the Women's Educational and Industrial Union of Boston, Gill moved to that city and established the first vocational placement bureau for college women. The problems experienced by educated women in finding suitable employment had first come to her attention during her work with nurses, and her experience as dean of Barnard and as president of the Association of Collegiate Alumnae had sharpened her sense of the need for organized assistance in that field. In 1911 she joined the staff of the University of the South in Sewanee, Tennessee, where she engaged in employment services for three years. She was similarly employed at Trinity College in Durham, North Carolina, in 1914–15. During World War I she served as a special agent in field organization for the U.S. Employment Service of the Department of Labor. In 1919 she became an educational worker at Pine Mountain Settlement in Kentucky, and in 1922 she became a housemother and teacher at Berea College in Kentucky.