• September 3 – The Dillon rodeo began.
• September 4 – Labor Day (which was not widely celebrated due to the war).
• September 5 – The schoolhouse in Richey was damaged by hail, forcing classes to be held in the jail.
• September 11 – Joseph T. McGeever, former Bobcat football star, was killed in action in France at age 26.
• September 14 – Six hundred state firemen met in Polson.
• September 20 – The William Clark Chalet in Butte, built in 1885 as the copper king’s summer home, was destroyed by an electrical fire.
• September 22 – One airman died when his light plane crashed on Mt. Wright, near what would become the Bob Marshall Wilderness.
• September 23 – Fire destroyed several historic buildings in Zortman.