Elmendorf began her library career in 1877 working for the Young Men's Association of Milwaukee. To develop a basic knowledge of library theory and practice, she read the 1876 report of the U.S. Bureau of Education, . About 1878 she started working at the Milwaukee Public Library, working her way up to deputy librarian in 1880 and finally head librarian in 1892. In 1896, at age 41, Elmendorf resigned her position to marry Henry Livingston Elmendorf. He was librarian of the public library in St. Joseph, Missouri, and vice-president of the AmericanLibrary Association. Henry Elmendorf soon took a position in Buffalo, New York, as the librarian of the Buffalo Public Library, where he worked until his death nine years later. During these nine years Theresa Elmendorf acted as his silent partner and was an administrative advisor. She worked on many library programs, including the Buffalo Plan, a program to help the city schools with library services, and the Descriptive Catalogue of the Gluck Collection of Manuscripts and Autographs in the Buffalo Public Library. Most notable in this collection was Mark Twain's Huckleberry Finn manuscript. She was also editor in 1904 for the American Library Association's Catalogue of Books for Small Libraries and an author of Classroom Libraries for Public Schools. Elmendorf became president of the New York Library Association in 1903 and 1904. When her husband died in 1906, the board of trustees of the Buffalo Public Library made Elmendorf vice-librarian, a position she held for the next 20 years. In 1911 she became the first female president of the American Library Association. She was in this position from May 24, 1911, to July 2, 1912.
Elmendorf retired in 1926. After this she wrote, edited and published many bibliographies and reading lists, including some specifically on poets and poetry. She worked on these until her death in 1932. In 1951 Library Journal selected Elmendorf for its Library Hall of Fame. Mary E. Hazeltine of the University of WisconsinLibrary System wrote of Elmendorf, "Many librarians in important positions today have carried on because she awakened their appreciation of books and opened up for them insight into new realms. She was a stimulating guide and a vitalizing teacher, as well as a great librarian."