In August 1853, Mattson led a group of several hundred Swedish immigrants to settle in Goodhue County, Minnesota.
[2] At the start of the American Civil War in 1861, Mattson raised a company of Swedish and Norwegian immigrants to fight for the Union.
He started with the Saint Paul and Pacific Railroad, where he worked as a protection agent to greet Swedish and Norwegian arrivals in Chicago.
[2] Mattson returned to Sweden as representative of the Minnesota Immigration Board in 1867 and 1868 to recruit settlers, a successful undertaking.
He wrote for Swedish American newspapers; he encouraged immigrants to write letters to friends and family in Europe, and he published pamphlets about the benefits of Minnesota.
During the course of his life, Mattson founded several Swedish newspapers in Chicago and Minneapolis, including the Minnesota Stats Tidning.
Scandinavian influence in Minnesota was growing, and the Republican Party sought Swedish and Norwegian immigrants as candidates for office.
President James A. Garfield offered him a diplomatic post because Mattson had become one of the most prominent Swedish Americans in U.S. politics.