Grannis was notable as the longtime Librarian of the
Grolier Club, a role which she held from 1906 to 1944. She began her career with the club as a cataloger and assistant to her predecessor,
Henry Watson Kent, in 1904. Upon Kent's retirement in 1906, Granniss became Librarian. In her early years, she was mentored by bibliographer
Beverly Chew. In 1917, she supervised the club's move to its new headquarters at 47 East 60th Street, New York, and directed the installation of its collections in the new building. At its new location, Granniss reportedly worked to make the library more open, inviting "non-members, including women and outside scholars" to use the reading room. In 1928, she travelled to Europe with her friend Henrietta C. Bartlett. Though it was a pleasure trip, she was authorized by the Grolier Club Library Committee to make purchases for the club while she was abroad. In 1935, a reception was held in her honor to celebrate her thirtieth anniversary as the Librarian. Over her career, she authored over twenty books and articles, and organized several exhibitions and their corresponding catalogues for the club. She was in frequent correspondence with other scholars and librarians, and gave regular lectures on various aspects of book history. She was said to be modest, and reportedly never allowed herself to be photographed. (Mildred Abraham suggests that this reticence may be due in part to facial scarring from injuries sustained in the streetcar accident in 1917.) Despite her position and contributions, she was barred from membership in the Grolier Club (which remained a male-only organization until 1976), and was unable to attend most of its events. She retired from the librarian role in 1944 (with a pension of $100 per month, and a cash gift of $1497 from Club members), and holds the distinction of having the longest single tenure as librarian in the club's history. George Leslie McKay, who had been appointed as her assistant in 1923, succeeded her as librarian. ==Later life and death==