Lewis Penwell was born July 24, 1869, in Diamond City, Montana Territory, the son of Oscar E. and Anna Lewis Penwell. Lewis attended school in Montana until he entered Racine College in Wisconsin at the age of sixteen. Following his studies he traveled throughout Europe for a year, returning to attend Columbia University Law School in New York.

After graduation in 1892, Penwell was admitted to the Illinois bar and worked in a Chicago law firm until January 1893, when he returned to Montana. He became a law clerk for Judge Thomas C. Bach and later went into partnership, first with former state Supreme Court Chief Justice Henry N. Blake and then with Judge R. Lee Word. Penwell later opened a private practice in Helena.

A prominent member of the Democratic Party, Penwell was elected as a representative to the 1897 Legislative Assembly. He served as an assistant prosecuting attorney for Lewis and Clark County and at one time was the chairman of the Democratic County Committee. He also worked for many years as the state collector of internal revenue. Penwell retired in June 1947.

Lewis Penwell also made substantial investments in the sheep industry and at one time was reputed to have one of the largest sheep herds in the world. Penwell revolutionized the sheep industry in Montana by placing the sheep he owned with ranchers throughout the state. These ranchers cared for the sheep, and once a year representatives of Penwell's operation collected the wool. The ranchers received a portion of the profits from the sale of the wool.

Dissatisfied with the federal government's policies concerning the Northwest, Lewis Penwell joined, and in 1911 was elected president of, the Northwest Development League. The League, dedicated to "the exploitation of the resources and opportunities" of the Northwest, worked to counteract the movement of settlers to Canada by enticing them to live in the Northwestern United States instead. The League held two meetings in 1911. The first, the Northwestern Development Congress, was held in Seattle in June, and the second, the Northwestern Lands Show (or Land Products Show), was held in December in St. Paul. The December meeting was preceded by the Western Governors' Special Tour, a railroad trip to the East made by the governors of Western states to try to bring the two sections of the country closer together on policies and other matters. Lewis Penwell was the treasurer for this Western Governors' Special.

In 1898 Lewis Penwell married Clare Avery, a native of Minnesota and daughter of F.D. Zickrick and Nancy Galligan. Clare Penwell was active in many Helena organizations, including the Students' Musical Club and The Current Topic Club. She died in the early 1930s. Penwell remarried, in February 1939, to Susannah Walters. Lewis Penwell died on March 20, 1948, in Tempe, Arizona.

Fred Penwell, the only child of Lewis and Clare Penwell, was born in 1900 and became a civil engineer. His wife, Priscilla Ash Garvin, was the daughter of Sam Garvin, a Montana sheep rancher, and Eva Ash, an early teacher in the state. Fred and Priscilla Penwell had four children: George, J. David, Lewis, and Priscilla.