Series 1 contains diaries and manuscripts of John G. Carter. Most of his writings were on Native Americans and include topics such as Indian Sign Language, Fort Belknap, the Piegan Tribe of the Blackfoot Nation, and Native American ceremonies. His diaries and notes cover trips he made to Montana, North and South Dakota, Indiana, Wyoming, Idaho, Washington, and Vermont. Other trips include Cuba, the Panama Canal Zone, Norway, Sweden, Russia, Denmark, Germany, Switzerland, Holland, Great Britain, Scotland, Ireland, and Canada.

As an attorney with A. R. Serven, John G. Carter was involved with numerous legal cases concerning Native Americans. Series 2 contains the business and legal papers of his law career. Important law suits include the Blackfeet et al. vs. United States during the 1920s and 1930s in which the Blackfeet, Blood, Piegan, and Gros Ventre nations sued the U.S. concerning the Common Hunting Ground set up by a 1855 treaty; Okanogan et al. vs. United States of the 1920s in which the Okanogan, Methow, San Poeils, Nespelem, Colville, and Lake Indian tribes of Washington sued the U.S. Also included in Series 2 is correspondence, petitions, arguments, depositions, analysis of the cases, opinions of the court, reports, motions, bills, and contracts.

Throughout his career, John G. Carter was very interested and connected to Native American concerns. Series 3 contains manuscripts and publications he collected on Native American topics. Included are works on Indian home life, customs, ceremonies, and habitat. Also included is a typescript of Lt. James Bradley's history of the march of General Gibbons Column at the Battle of Little Big Horn, and two books on Native Americans from Carter's personal library that are annotated.

Series 4 contains Carter family papers, including correspondence of Senator Thomas H. Carter and newspaper clippings and papers concerning his death and clippings and material on Trinity College regarding Ellen Galen Carter.

Series 5 contains photographs mainly of Native Americans that John G. Carter collected during his career. Most of the pictures are of ceremonies performed at the Fort Belknap Reservation including the Fool Dance, Grass Dance, and Sun Dance. Also included are pictures of a "sham" battle between the Assiniboine and Gros Ventre, Medicine Man "Nosey" and Little Chief, and building a medicine lodge. Two family photographs of Thomas H. Carter and Hugh Galen are also found in Series 5.