After working as a high school teacher for a short time, Jackson moved to Washington, D.C. and began working with the War Department. Because of World War II, Jackson had to leave her teaching position in the segregated southern schools. But even in Washington, "colored only" signs were still fairly common and "the racial division of labor that segregation imposed did not promise much for the young black woman from a small black college." Nevertheless, in 1944 she was offered a position in the Military Archives Division of the National Archives and Records Administration, where she became a self-taught expert on records pertaining to the War Department, the U.S. Army and Navy, the Adjutant General's Office, the Engineer Department, the Bureau of Colored Troops, and the Freedmen's Bureau. She was one of the first African American professionals to be hired by the National Archives in Washington, D.C. Here, she specialized in western, military, social and African American topics. She became one of the most knowledgeable historians and archivists of American life. In the 1950s and 1960s, many Americans were stirred by the Civil Rights Movement and were inspired to study American history. Pushed by racial equality and injustice, these new historians turned to Jackson, where the lines queued through the archives to her desk. The historian Ira Berlin credits Jackson for supporting a transformation in United States historical thought, writing,
a new generation of historians, understanding that transformation of the American present required the transformation of the American past, took up the challenge of rewriting our history. When they arrived at the National Archives, Sara Jackson was ready...She directed them, gently through the power of suggestion and, then, if they did not get the point— well, Sara had her way. Armed with knowledge squeezed from the records, scholars began to write a new history of the United States. It is no exaggeration to say that history rests, to a considerable measure, on the work of Sara Jackson, for Sara Jackson was a great teacher.
He later dedicated a volume in his Freedom series to Jackson with the inscription, "To Sara Dunlap Jackson: Archivist Extraordinaire." In 1968 she began working with the letters of many prominent Americans at the National Historical Publications and Records Commission, a division of NARA. She retired from the agency in 1990. On April 19, 1991, Jackson died of cancer at her home in Washington, D.C. When she passed away, she was serving a term on the Western Historical Quarterly Board of Editors.
