The City of Longmont is a Home Rule Municipality in Boulder and Weld counties of the U.S. state of Colorado. Longmont is located northeast of the county seat of Boulder and 33 miles north-northwest of the Colorado State Capitol in Denver. Longmont's population was 86,270 at the time of the 2010 U.S. Census. Longmont is the 13th most populous city in the state of Colorado. The word "Longmont" comes from Longs Peak, a prominent mountain named for explorer Stephen H.