In mid-1856, J. Wycroft laid claim to land west of Honey Lake. In November, it was taken over by Thomas Mitchell, but neither had made any improvements and soon the claim was abandoned. In May 1857, Malcolm Bankhead acquired the abandoned claims and built a two-story log cabins where he offered rooms. By the early 1860s, others arrived and opened businesses, and a small community had developed. Until then, it had informally been known as Bankheads, but when it received its post office in 1861 it took on the name Janesville after Bankhead's wife. Janesville continued to grow, and in 1864 was even considered for the Lassen County seat. In 1872, Dennis Tanner built a new two-story hotel to replace Bankhead's original cabin, and in September 1882 Hiram Dakin & Hiram McCellan opened a flour mill that operated until 1918.

In 1914, following the arrival of the Fernley & Lassen Railway in Honey Lake Valley, there was some sentiment that there were too many "ville" towns in the valley, and Janesville was renamed Lassen as a result. The name was not well received, and the locals still referred to the town as Janesville despite the post office being called Lassen. A petition was sent to the Postmaster General but denied; a second petition in 1923 was accepted and Janesville's name was restored.

For the next several decades, Janesville has maintained a modest existence. Though it is home to over 2000 people, a quiet Main Street with a handful of historic buildings gives a sense of a much smaller community.