The Mary McCornack Thompson Diaries date from 1887 to 1962 and are arranged into two series: Diaries and Correspondence. The bulk of the collection consists of 90 journals that contain detailed accounts of Mary McCornack Thompson's work as a Presbyterian missionary and teacher with the American Board of Commissioners for Foreign Missions in South Africa. During her 43 years as a missionary Thompson worked briefly at the mission station at Esidumbi in South Africa, but she spent most of her time at the Mount Selinda mission in the Melsetter region of Rhodesia ( Zimbabwe). In the diaries, Thompson wrote of her daily activities as a missionary, including building and expanding the mission, encounters with locals, learning Zulu, wildlife, meeting other missionaries, teaching and praying. These detailed entries offer a glimpse into the social conditions, race relations, and native cultures of various South African regions. Thompson also recounts her many travels throughout Africa, Europe, Asia, the United States, and Canada. Included in the collection is one folder of correspondence, mainly from William L. Thompson (Thompson's husband) regarding the collection and the transfer of Mary's diaries to Oberlin College.

The Diaries Series documents Thompson's almost daily activities between the years of 1887-1933, spanning all five of her missionary trips to Africa. Volumes 1-6 describe her first missionary trip (1887-1899), detailing her preparations for travel to Africa, her arrival, and her first encounters with native Africans. During this time Thompson married another missionary, William L. Thompson, and together they traveled for four months, mostly on foot, from South Africa to Rhodesia (Zimbabwe). They settled at Mount Selinda, which would be their home in Africa for the next forty years. Volumes 6-8 describe Mary Thompson's visits to the United States between her missionary trips, including taking cooking and photography classes, and traveling around the U.S.

Volumes 8-35 detail her second trip to Africa (1901-1910), during which time the mission at Mount Selinda began to expand rapidly. Thompson often writes about elections at the mission, as well as prayer services and sermons. She occasionally mentions world events such as the explosion of Mt. Pelee in Martinique, the Russian Revolution, and the detention of Queen Wilhelmina of Holland. She also describes her experiences with local natives who teach her the Zulu language. Volumes 35-40 cover Thompson's trip back to the United States in 1910. She describes lectures and meetings, and discussions on the outbreak of World War I. Her diary entries become less frequent during her stay in the United States.

Volumes 40-57 span her third trip to Africa (1911-1917), and entries tend to be bit longer and more descriptive. On this trip volumes 44-49 were written in diary volumes entitled "Warriors of Africa," whose covers depict African natives, and volumes 52-55 in volumes bearing the title "Empire Exercise," portraying historical events. Volumes 57 and 58 describe Thompson's travels during 1916-17 (at the height of World War I) to Hong Kong, Japan, Canada, and the U.S. Volumes 59-60 recount her time back in the United States; much of the content revolves around religious and political meetings on World War I, and the 1918 U.S. midterm elections..

Volumes 61-77 detail her fourth trip to Africa (1919-1925), and volumes 78-89 her fifth and last trip to Africa (1926-1932). Volume 80 does not begin until page 92, and is filled with various writing; some entries appear to be copies of diaries of historical figures. The diary entitled "Notes on Work at Moody Bible Institute" contains lecture notes, thoughts, scripture quotations, and observations by Thompson while attending a higher-education Christian organization, Moody Bible Institute, in Chicago in 1918, between her third and fourth missionary trips to Africa.

The Correspondence Series contains six letters regarding the collection and transfer of Mary McCornack Thompson's diaries after her death in 1936. The first five letters are from by William L. Thompson (Thompson's husband), to his nieces Margaret and Jay Urice, who are locating and collecting Mary's diaries. The sixth letter is from Jay Urice to Mr. Julian Fowler, a librarian at Oberlin College, about having Mary's diaries sent to Oberlin.