Brynild Anundsen was born at Skien in Telemark, Norway. He was the son of Anund Brynildsen Brækkejord (1817–1903) and Maren Amundsdatter Berberg (1812–1883). Anundsen immigrated to the United States during 1864 settling first into La Crosse, Wisconsin. He subsequently served as a private in the Union Army during the American Civil War between 1864 and 1865.
Anundsen, who had learned the printer's trade in Norway, worked for the Norwegian language newspapers Emigranten in Madison, Wisconsin, and Fædrelandet og emigranten of La Crosse, Wisconsin. In 1867 he moved to Decorah, Iowa, where he established Ved Arnen, a Norwegian language literary magazine in 1866. In 1868 he took over the printing of the journal, Kirkelig Maanedstidende for the Norwegian Synod, an arrangement which continued until 1877. He started the newspaper Fra Fjærnt og Nær in 1869. 
On September 18, 1874, Anundsen started Decorah Posten. It was a well-edited newspaper, its size and familiar format developed gradually. The publisher avoided areas of political and religious controversy, which had destroyed so many earlier papers. Anundsen made a success of the Decorah Posten and went on to establish the Anundsen Publishing Company. Anundsen was a founder of the Norwegian Society in America (Norwegian: Det Norske Selskab i Amerika). In 1878 he participated in the founding of the Norwegian Society in Decorah and 1903 The Norwegian Society of America. In 1895, he was co-founder and first president of the Norwegian-Danish Press Association of the United States.  
In 1865, Anundsen was married in La Crosse, Wisconsin, to Esther Mathilde Charlotte Hofstrom (1838–1889), a native of Sweden. Following the death of his first wife, he married Helma Beatha Hegg (1872–1951) in 1901. He was the father of Fred Anundsen (1872–1931) and Brynjolf Anundsen (1902–1984).
In 1906 he represented the state of Iowa at the coronation of Haakon VII of Norway and Queen Maud and was made a Knight First Class of the Royal Norwegian Order of St. Olav by King Haakon VII. Anundsen was an active member of the United Lutheran Church of Decorah.
- "Anundsen, Brynild". Lutheran Cemetery. Winneshiek County, Iowa. Retrieved May 1, 2017.
- B. Anundsen, Biographical (Past and Present of Winneshiek County, Iowa. Volume II. pages 5–7. 1913) "Archived copy". Archived from the original on 2008-10-10. Retrieved 2009-10-20.
- "Brynild Anundsen". Store norske leksikon. Retrieved August 17, 2015.
- Brynild Anundsen, Publisher of the "Decorah-Posten" Norwegian-American newspaper (The Promise of America. National Library, Oslo)
- Odd Lovoll. "Brynild Anundsen, Avisleder". Norsk biografisk leksikon. Retrieved August 17, 2015.
- Decorah-Posten: The Story of an Immigrant Newspaper (Odd S. Lovoll, Norwegian-American Historical Association. Volume 27: Page 77)
- Norsk-Amerikanernes Festskrift 1914 (The Promise of America)
- The Biographical Dictionary of Iowa (The University of Iowa Libraries)
- Andersen, Arlow W. (1990) Rough Road to Glory: The Norwegian-American Press Speaks Out on Public Affairs, 1875 to 1925 (Balch Institute Press) ISBN 978-0944190029
- Brøndal, Jørn (2004) Ethnic Leadership and Midwestern Politics: Scandinavian Americans and the Progressive Movement in Wisconsin, 1890-1914 (University of Illinois Press) ISBN 9780877320951
- Øverland, Orm (1996) The Western Home: a literary history of Norwegian America (Norwegian-American Historical Association) ISBN 978-0252023279
- Øverland, Orm (2010) Norwegian Newspapers in America: Connecting Norway and the New Land (Minnesota Historical Society) ISBN 9780873517720
- Bailey, Edwin C. (1913) Past and present of Winneshiek County, Iowa: A record of settlement, organization, progress, and achievement
- Nelson, O. N. (1904) History of the Scandinavians and Successful Scandinavians in the United States (O.N. Nelson & Co.)