The Thelma Petersen papers date from 1926 to 2003 and measure 2.5 linear feet. The bulk of the collection consists of her diaries, which date from 1949 to 1998, although the years 1951-1954 are missing. The correspondence includes letters from friends and family members written to Thelma Petersen while she attended Iowa State Agricultural College in Ames, Iowa. Thelma Petersen's diary entries focus on her daily life: cooking, gardening, washing, and weather. In the front and back of most of the diaries Petersen records information about a variety of topics including visitors, money she tithed to her church, and her hair permanents. The diaries are mainly written in spiral notebooks and some contain calendars and clippings folded in the back of the notebook.

The Thelma Petersen papers also include the diaries of her mother Katie, who was born in 1874 in Coon Valley, Wisconsin, and died in 1972 in Shelby County, Iowa. Katie Petersen's diaries include details of her daily life as a wife and mother on an Iowa farm.

Four photographs of family members complete the collection: a 1946 portrait of Katie and Hans Petersen on their 50th wedding anniversary, photos of Thelma and Archie Petersen in 1980, and an informal portrait of Thelma and her seven siblings in 1987.