Peter Lassen likely arrived in America in the early spring of 1831, probably landing in Boston, Massachusetts, where his passport listed his destination and where he first encountered the many languages of his new country. In Boston he worked as a blacksmith, stayed only a couple of months, and after arriving in America changed his name from Larsen to Lassen. Little is known about his movements from 1831 to 1840 as he traveled from place to place on his way toward California, seeking places where he could explore, work, be effective, have a beneficial effect, and perhaps settle. His next known stop was Keytesville in Charlton County, Missouri, where he lived about 7 or 8 years, worked again as a blacksmith, joined Warren Lodge No.74, and first met Johan August Sutter, who later helped him in California. After selling his land, Lassen headed west in the spring of 1839 with a party of 11 men and 2 women; when they reached Fort Hall north of Salt Lake, the two women remained there because they were exhausted, and the party then increased by 27 trappers. The route took them to The Dalles on the southern bank of the Columbia River, then by boat downriver to Fort Vancouver near the meeting of the Columbia and Willamette rivers, and south along the Willamette to Camponit, now Oregon City, where reports that snow had begun in the Sierra Nevada made crossing impossible, so Lassen and his companions stayed there for the winter while he waited to continue on to California.