Shortly after graduating from college, McCormick started working as a substitute at NYPL. After a year of substituting McCormick took a leave of absence to attend the NYPL Library School and completed the one-year certificate course in 1924. She then received a regular appointment at the Fordham Branch.
McCormick worked in the Extension Division, 1926-1936, and for a short period in 1936 she was the Acting Superintendent of the Book Order Office. McCormick was promoted to Branch Librarian in 1937 at the Epiphany Branch. She later served two years as head of the Aguilar Branch and finally headed the Bloomingdale Branch for 20 years, 1941-1961, before retiring.
I was once told by a librarian at the Bloomingdale Branch that Emily McCormick was known for taking staff members of the branch with her on trips to Europe. This would certainly be an interesting fact, if true. While her father was a successful businessman, it is not clear that McCormick would have had enough money to pay for the vacations of staff members. In fact the ship passenger lists available through Ancestry.com do not support this story. While McCormick did travel to Europe, as did other Branch Librarians, I found that there was no evidence that any NYPL staff members accompanied her on these trips. This story is apparently an oral tradition that got mangled over the years.