In 1851, he immigrated to the U.S. with a friend. He settled on a farm in an established Swedish colony in Illinois in 1853 and brought his family from Sweden to join him. But Mattson did not find Illinois promising. He left in search of better land just a few months after settling there.
In 1857 due to panic Mattson was financially ruined. He served with distinction as a colonel in the American Civil War (1861–1865) and then returned to Minnesota.
After returning to Minnesota, Mattson began his work as an immigration booster. First he worked with the St. Paul & Pacific Railroad Co, where he was a protection agent to greet Swedish and Norwegian arrivals in Chicago.
Building on his experience, in 1866, Mattson proposed the creation of a state Board of Immigration. The board would recruit immigrants to homestead land in Minnesota. Until the 1880s, immigration to the U.S. was regulated by states rather than the federal government. In 1867, Governor William Marshall established the board and named Mattson to be its first secretary. The state was especially interested in recruiting Scandinavian immigrants, who were considered to have good moral character.
As a booster, Mattson promoted Minnesota in Sweden and Norway. He also promoted the state to Scandinavian immigrant communities in the Eastern U.S. Mattson recruited immigrants to Minnesota by several means. 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 Statstidning.
Member Minnesota Board Immigration, 1867-1872.