Boulder County’s history began long before the formation of its territorial government in 1861. The mark of human settlement on this region runs the gamut, from archaeological finds and tipi rings left by early nomads, to satellite tracking devices built by the local operations of national industries. The human imprint upon the county has been as diverse as the geological landforms that define our surroundings.

Between the 1803 Louisiana Purchase and Colorado’s admission into the Union as a state in 1876, present-day Boulder County was included within the jurisdictional boundaries of a succession of territories: Louisiana, 1803-1819; Missouri, 1819-1854; Nebraska and Kansas, 1854-1861; and Colorado, 1861 until statehood in 1876. Today, Baseline Road, running west to east through central Boulder, is a reminder of the territorial era, as the road was built along the 40th parallel, which once separated the Nebraska and Kansas territories and now forms the boundary between the two states.

In 1849 the discovery of gold in California had given new impetus to westward migration. The first recorded local encampment of these new settlers was at Red Rocks (now known as Settlers' Park) in Boulder Canyon, on October 17, 1858. The party of about 20 Nebraskans in search of gold was led by Captain Thomas Aikens. The Arapaho and Cheyenne, who had been granted a land area which included Boulder County, were fearful of the intruders, but Aikens was able to converse with Niwot, Chief of the Arapaho, and establish a friendly relationship with the indigenous people. Much credit for the initial peaceful coexistence of the local inhabitants and Boulder County settlers was due to Chief Niwot (meaning "Left Hand"), who was admired and respected by the settlers. A bust of Chief Niwot is displayed on the Boulder County Courthouse grounds, an honor to his contribution in the settlement of this area. The hamlet of Niwot, platted in 1875 along the Colorado Central Railroad line, was also named in honor of the Arapaho Chief who figured so prominently in county history.

In 1861, the Colorado Territory was established by Congress, replacing the powerless Jefferson Territory. The federal government also signed a new treaty with the native residents, pushing the tribes into the arid, barren land between the Arkansas River (which flows through present-day Cañon City and Pueblo) and Sand Creek. As military troops were removed from Colorado Territory to join the Union effort in the Civil War, raids on settlers increased. The settlers erected a number of small forts, one of which, Fort Chambers, was built near the Valmont settlement. From there, in November 1864, Captain David Nichols and 100 volunteers left to help "settle the Indian problem." They joined other troops at Fort Lyon and, under the command of Colonel John Chivington, participated in the bloody massacre at Sand Creek. Three years later, after many battles, the Arapaho and Cheyenne, who had long inhabited Boulder County, were permanently removed from the territory and relocated to Oklahoma. Despite Aikens' and Niwot's amicable beginning, the situation tragically ended in war and, ultimately, the loss of homeland for the native peoples.

Between 1890 and at the onset of World War I, Boulder County felt the impact of events taking place well beyond its borders. With the financial panic of 1893, the silver market all but collapsed. William Jennings Bryan—and his plan for free and unlimited coinage of silver—was defeated in the 1896 presidential election, despite carrying Colorado 161,269 to 26,279. Boulder County coal mines had their troubles as well in the form of strikes and major gas explosions. The coal fields and railroads throughout the state were being brought under corporate control, bringing new conflicts and making locally-based competition difficult. The county population swelled with new arrivals from China, northern Europe and the British Isles, bringing mining experience with them. By the turn of the century, there was an influx of immigrants from southern and Eastern Europe to work the coal fields; immigrants from Japan and Mexico joined the German and Scandinavian farmers in the area around Longmont. The immigrants initially settled in ethnic colonies, but in time, these varied cultures were interwoven to form the rich tapestry of Boulder County's own heritage.

Life at the turn of the century was not easy. Divisions and disasters were common. There was passionate dissension over alcohol and its permissibility in the community ("wet" versus "dry"); miners faced off against mine operators; flood and fire ravaged parts of the county. Those who came to Boulder County seemed to have a spirit of resilient determination that eclipsed the adversity they faced. Part of the resilience no doubt stemmed from the potential of the land and its resources. When one resource played out, another was waiting to be explored or developed. Tungsten replaced gold as the most profitable resource being mined in the mountains. On the eastern plains, oil was discovered, and derricks began popping up on local farmlands. The creation of Rocky Mountain National Park brought a new surge of tourism. The strength of commercial activity was aided by the extensive system of railroads in the county, including the rail line constructed by David Moffat in the southern part of the county that spanned the Continental Divide. Progress, however, was not a straight and smooth road. A violent strike at the Columbine Mine east of Lafayette rocked the coal mining industry. The depression of the 1930s and the dust bowl conditions of that decade affected the entire region. Though gold mining experienced a temporary revival when gold was revalued, with the advent of World War II, gold and silver mining ceased, replaced by tungsten and fluorite as "essential" minerals. These too, declined with the end of the war.

After the Second World War, a new kind of growth came to Boulder County—this time tied less to natural resources than to human resources. Scientific research and development, both public and private, began to locate in Boulder County. Among these new institutions was the Commerce Department's National Institute of Standards (NIST), which came to the City of Boulder in 1954. In the decades that followed, the National Center for Atmospheric Research (NCAR) and the National Oceanic and Atmospheric Administration (NOAA) followed suit, contributing to the area’s reputation as a major center of scientific and environmental research. During that same era, the Boulder-Denver Turnpike opened, improving transportation between Boulder and Denver. Construction of the Turnpike spurred growth along its corridor, including the planned community of Broomfield, which in 2001 separated from Boulder County to become its own county.

The 1950s and 60s saw local industry expand to include small and large developers of electronic and computer technology. IBM and Ball Aerospace opened facilities in Boulder County, and to this day IBM remains one of the county’s largest employers. By the late 1970s, Boulder County would be regarded as the "silicon valley" of Colorado and still continues to attract businesses from far and wide. Other industries have flourished in the county as well: enterprises from breweries to purveyors of organic food have thrived in Boulder County, drawn here and energized by its entrepreneurial spirit, and its stunning natural surroundings and preserved open space.

The origin of what today is one of the region’s preeminent open space programs can be traced back to the City of Boulder’s Blue Line charter amendment. In the late 1950s, voters, concerned with the encroachment of residential developments into the foothills and beyond, overwhelmingly approved a measure that would prohibit the city from providing water to new residences above the “blue line”—a boundary at an elevation of roughly 5,750 feet, running north from Eldorado Springs. This effort inspired additional organizations committed to the preservation of open space within Boulder County. In 1967, the City of Boulder passed the nation’s first sales tax dedicated to the acquisition of open space. Since then Boulder County has followed suit with a number open space taxes of its own. Today Boulder County has conserved over 100,000 acres in agricultural and recreational land for future generations, and the program has become a model for other communities that are concerned with unrestrained development.

In 2010, the County also experienced a devastating mountain fire. The Fourmile Canyon Fire raged from September 6th through the 16th, consuming 6,181 acres and destroying 172 structures, 169 of which were homes. At the time, the fire was the most expensive in Colorado history with insurance costs in excess of $217 million. The fire danger remained high for several years as the county entered a severe drought. According to the US Drought Monitor, Boulder County was in an “extreme” drought along with 70% of Colorado during the summer of 2012. Dry, hot weather left agricultural lands in poor conditions, negatively affecting crop yield. A wet spring and substantial snowpack around the turn of 2013 brought water levels back to a reasonable level, allowing the county’s watersheds to replenish.

Colorado weather is known for its erratic behavior. The 2012 drought was followed by a devastating flood in September of 2013. Boulder County experienced an unprecedented rainfall which led to flash floods along every major drainage location in the county. The flooding cut off several communities from aid for many days, and extensive road and bridge damage left several mountain towns inaccessible for months; 219 homes and 95 manufactured homes were destroyed. Tragically, four residents lost their lives. In the days immediately following the flood, 1,102 individuals were evacuated from the mountains by air, making it the 2nd largest airlift after Hurricane Katrina. Roughly 30 miles of county roads were destroyed, 150 miles of roads damaged, and 36 county maintained bridges either suffered damage or were destroyed. It was determined that 164 acres of county-owned open space and 111 miles of trails were damaged. The great flood of 2013 was declared a federal disaster, and recovery continues today.

On December 30, 2021, Boulder County faced an unprecedented disaster in the form of a fast-moving wildfire that struck the densely populated areas of Louisville, Superior, and Unincorporated Boulder County. Boulder County has partnered with the affected municipalities to address debris removal and is coordinating the effort on behalf of the State.

Natural disasters are a part of our landscape, but Boulder County remains a resilient community which adapts and learns in the face of disruptive shocks and stresses. This tenacious adaptability transcends into many areas as our demographics continue to change. Although the population has grown, urbanization has not eradicated all the signs of Boulder County's early history. Boulder County has active agricultural and mining preservation projects. Landmark preservation efforts are evident in every community, and local historical and archaeological societies help perpetuate a sense of the past. One might argue that marks of human settlement have not always been positive, but the lessons of the past continue to guide us into the future. The human energy, entrepreneurial spirit and optimism which arrived with the first settlers to Boulder County will continue to be the driving force of human activity in the area.