[5] Lassen used several different spellings while living in Denmark and in Copenhagen, sometimes preferring to be called Peter Larsen Farum, after his birthplace.

[10] The expedition Lassen joined was made up of missionaries and adventurers, some on their way to California; others headed to the Columbia River basin in Oregon.

[11] Lassen arrived at Sutter's Fort in August 1840, with William Wiggins, who had traveled with him from Westport (now Kansas City, Missouri), over land and by sea for nearly fifteen months starting in May 1839.

John Sutter happily welcomed the men and traveled with them from his colony, known at the time as New Helvetia (meaning New Switzerland), to Yerba Buena and Alviso, California, in a small sailboat.

[10] While it is likely he was working as a blacksmith during his early years in California, Lassen traveled to Santa Cruz in 1841, where he oversaw the construction of a sawmill owned by Isaac Graham on Zayante Creek.

[10] In July 1842, the men arrived at "Rancho Campo de los Franceses" but just missed connecting with French-Canadian trappers who worked for the Hudson's Bay Company.

[10] Lassen pushed on with his livestock, and instead spent the next few years camped in a hut along the Cosumnes River, working as a travelling blacksmith.

Planning carefully, Lassen also dispatched agents to divert forty-niners onto the cutoff and to set up trail advertisements (including a signboard at Lassen Meadows, where the Applegate Trail branched off from the Humboldt River) with the reassuring message that the diggings were a mere 110 miles ahead ... [A later] emigration trustingly followed their lead, many foolishly discarding surplus provisions on the assumption that only 110 miles remained.

[24] William Weatherlow, the Susanville resident, leader of the Honey Lake Rangers, and close friend of Isaac Roop was known to be in the area and was leading the prospecting party Lassen, Clapper, and Wyatt were in search of.

Weatherlow was thought by local Indian Agent Major Dodge to have been the murderer under order from Isaac Roop for political gain as Lassen was a prominent and leading candidate for the provisional governorship of the forming Sierra Nevada Territory.

Michael Witney portrayed Lassen in a 1968 episode, "The Other Side of the Mountain", of the television anthology series, Death Valley Days, hosted by Robert Taylor.