Series 1: Correspondence, 1922-1977, is made up of correspondence between Lillian Cooper, her family, friends, and professional contacts. The bulk of the correspondence is from the 1930s through the 1940s. Topics in the correspondence include labor organizing, socialism, writing, and the experience of being Jewish in the United States. Prominent correspondents include dancer Maudelle Bass Weston, Gertrude W. Klein, one of the first women elected to the City Council of New York, Gertrude M. Ruskin, Comptroller of the United Service for New Americans and Analyst for the Displaced Persons Commission, author Sylvia Chatfield Bates, artist Michael Baxte, German philosopher Beate Berwin, and German writer Gustav Regler.

Correspondence is also present in the Administrative Files and Herman Kobbé papers, especially between Cooper, Kobbé and Laney Stephens.

Series 2: Writings, 1933-1983, contains published and unpublished works, mostly short fiction pieces and poetry, by Lillian Cooper. Cooper also published and submitted pieces under the pen name Joan Kinner. The works are typewritten and handwritten in notebooks and calendar books. Pieces submitted to specific journals or conferences, or for writing courses are in folders titled with that journal, conference, or course.

Series 3: Administrative Files, 1935 - circa 1980, includes licenses, teaching certificates, and other paperwork regarding Lillian Cooper's teaching career, courses she took, and conferences she attended; resumes; a handwritten will from 1969; and immigration documents related to William Hirschel Eichhorn whose relationship to Lillian Cooper is unknown. The bulk of this series consists of correspondence and paperwork related to the Holly Point Project, a collaboration between Lillian Cooper and Herman Kobbé to build a children's development center on their land around Lake Murray, South Carolina including a boarding school, programs for parents, and a camp for city children. Related to these files are files regarding Cooper and Kobbé's break-up around 1948.

Series 4: Herman Kobbé papers, 1935-1947, is made up of correspondence between Herman Kobbé and his wife Helen (Lili) Kobbé (née Metcalfe) and Lillian Cooper, his partner from about 1935 to 1948. The correspondence between Cooper and Herman Kobbé represents the early days of their relationship, love letters and plans to see each other, often at their apartment at 209 Washington Park. Correspondence between Lili Kobbé and Lillian Cooper regarded Lilian's relationship with Kobbé and work that both women were involved in. Also included is a folder of documents related to Kobbé's time in Mexico (with Cooper) in 1938 and some writings and drawings by him.

Correspondence to both Cooper and Kobbé is also included in Series 1: Correspondence. Correspondence between Cooper and Kobbé is also included in Series 3: Administrative Files.

Series 5: Photographs, circa 1905-circa 2000, includes photographs of Lillian Cooper, her family, and friends. The scrapbook contains mostly photographs and photo booth snapshots of Lillian Cooper. Other photographs include both candid and professional portrait photographs of friends and family. The "Cooper-Pazeian" photographs mostly depict Lillian's sister Gertrude and her husband Hrant Pazeian's family. Many of the people in the photographs are unidentified.