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.