Peter Lassen was born on October 31, 1800, in Farum, Denmark, then a small village about 15 miles northwest of Copenhagen. He was the son of humble parents, Johanne Sophie Westergaard and Lars Nielson, who in church records was called a farm laborer, day laborer, or smallholder. His family name was not originally Lassen; in keeping with a common Danish patronymic practice, he was first called Peter Larson, meaning son of Lars. While living in Denmark, he spelled his name in several ways, including Peter Larson, Peter Larssen, and Peter Larson Farum, and in Copenhagen passport records from 1830 he was registered as P.L. Farum. While living in Copenhagen, he appears to have preferred the name Peter Larsen Farum, likely to distinguish himself from other men with the common name Peter Larson. After coming to America, he changed his name again and thereafter was known as Peter Lassen.