In 1917, Worthington and Captain Frederick S. Keen of Seattle took over the assets of bankrupt Gerlinger Motor Car Company of Portland and Seattle. Worthington and Keen were in the marine finance business and were Gerlinger's landlords in Seattle. They formed the Gersix Manufacturing Co. to continue making a six-cylinder truck. In 1922, Gersix made 53 trucks at its factory on Fairview Avenue at Valley Street.
Under the new name, the company moved to 506 Mercer Street and later to 1263 Mercer Street. Trucks and motor coaches were assembled in individual bays rather than on a conventional assembly line. Kenworth was first to offer the diesel engine as original equipment in a truck.
During World War II, Kenworth built heavy-duty wreckers for the Army, and sub-assemblies for the Boeing B-17 bomber.
After Kenworth President Philip Johnson died in 1944, Paul Pigott (1900-1961) purchased the company on behalf of Pacific Car and Foundry Co. In January 1946, Pacific Car purchased the Fisher Body plant near Boeing Field as a new home for Kenworth.