Mithun has worked on a wide variety of languages from a wide variety of language families, but specializes in Native American languages. She began her career with extensive fieldwork on Iroquoian languages, especially Mohawk, Cayuga, and Tuscarora, She has also worked in California on Central Pomo and the Chumashan languages, on Central Alaskan Yupik, and on the Austronesian language Kapampangan.
Mithun compiled a comprehensive overview of Native American languages in The Languages of Native North America. A review on the Linguist List describes the work as "an excellent book to have as a reference" and as containing "an incredible amount of information and illustrative data." The work is a bipartite reference organized firstly by grammatical categories (including categories that are particularly widespread in North America, such as polysynthesis), and secondly by language. In 2002 the volume won the Leonard Bloomfield Book Award, which is awarded yearly for the best book in linguistics.
She was the founding president of the Society for Linguistic Anthropology in 1983. From 1999 to 2003 she was president of the Association for Linguistic Typology. She is a member of the Norwegian Academy of Science and Letters.
Selected works 
- 1999 The languages of native North America 
- "UCSB Linguistics Marianne Mithun". Retrieved 2009-06-04. 
- "Society for Linguistic Anthropology Records". American Philosophical Society. Retrieved 2008-07-17.
- Mithun, Marianne (1999). The languages of native North America. Cambridge, UK: Cambridge University Press. ISBN 0-521-23228-7.
- "LINGUIST List 12.707: Mithun, Languages of Native North America". Retrieved 2009-06-04.
- "LSA: About LSA". Retrieved 2009-06-04.
- "Marianne Mithun: Curriculum Vitae" (PDF). Retrieved 2009-06-04.
- "www.linguistic-typology.org: List of officers" (PDF). Retrieved 2009-06-04.
- "Gruppe 5: Filologi og språkvitenskap" (in Norwegian). Norwegian Academy of Science and Letters. Retrieved 10 January 2011.
- "UCSB Linguistics Marianne Mithun". Retrieved 2009-06-04.