- U.S Highway 6 through Clear Creek Canyon is constructed
- Golden Post Office at 619 12th Street is built. Designed by Lewis Simon and modeled after other post office buildings of the era, it is built with funds from the Public Works Administration.
- Berthoud Hall on the campus of the Colorado School of Mines opens. Designed by Temple Hoyne Buell, it is one of two New Deal buildings on campus. Named after the school's first geology professor and inaugural board member, Edward L. Berthoud.
October 17, 1940
Undersheriff Clarence Bunch Fugate is the first Jefferson County Sheriff's officer to die in the line of duty.
- The Federal Government contracts with Remington Arms Company to operate the Denver Ordnance Plant, the largest contract awarded by the federal government in Colorado up to that time. Construction begins in May, with operations beginning in October. Now the site of the Denver Federal Center.
- Coors introduces Coors Light beer
- Jefferson County's first stoplight installed at the intersection of Olde Wadsworth Boulevard and Grandview Avenue in Arvada
June 15, 1941
Red Rocks Amphitheater is formally dedicated.
September 7, 1942
Denver to Golden interurban car hits horse at a crossing, killing its 15 year old rider, Janet Lamoure of Arvada.
April 10, 1943
Two boys from Lakewood, Donald Matta, age 8, and Milo Flint, age 11, were robbed and then pushed off the top of Castle Rock in Golden by William Wymer. Wymer, age 16, was found guilty of the slaying of the two young boys and punished to life in prison.
Ruth W Quinn of Edgewater becomes the first woman to serve on a Jefferson County jury.
August 24, 1946
Flood from rainstorm near Idledale floods Bear Creek, killing woman at Morrison.
1947 to 1950
Denver Federal Center established on the site of the Denver Ordnance Plant.
June 7, 1948
Second Tucker Gulch flood hits Golden causing major damage.
Golden's iconic Welcome Arch is built.
June 3, 1949
Bill and Dorothy Harmsen open the Jolly Rancher Ice Cream store on Washington Avenue in downtown Golden. Later develop the world-famous Jolly Rancher hard candies.