During the 19th century, several explorers, including Zebulon Pike, Stephan Long, and later John Fremont, were commissioned to explore the Boulder area. One of Fremont's men named William Gilpin, who later became the first governor of the Colorado Territory, reported that there was gold to be found in Boulder, sparking interest in an area formerly considered unfit for settlement.

The Flatirons, slabs of sedimentary stone that resemble old-fashioned clothes irons, are located on the foothills west of the city. The third Flatiron bore a "CU," for the University of Colorado, in 50-foot (15 m) white letters until 1980, when the city painted over the initials to restore the natural look of the stone formation. CU students Dale Johnson and Robert Rowlands climbed the flatiron one night in November 1949 to paint the original "C." The "C" was removed and painted again, with a "U" being added in the 1950s. The letters are still faintly visible under the reddish paint used by the city.

In 1907, Boulder adopted an anti-saloon ordinance. Statewide prohibition started in Colorado in 1916 and ended with the repeal of national prohibition in 1933, although the city did not officially overturn its own anti-saloon measure until 1967.

In The Stand (1978), Boulder is the gathering point for some of the survivors of the superflu. King lived in Boulder for a little less than a year, beginning in the autumn of 1974, and wrote The Shining (1977) during this period.

In 1899, Tungsten was discovered in the mountains west of Boulder. The ore's value soared, especially after the United States entered World War I, greatly increasing demand. By that time Boulder County was the world's leading producer of tungsten ore.

A 1904 city ordinance made it "unlawful for any person to ride or drive within Boulder at a rate of speed in excess of 6 miles per hour."

Snowfall averages 88 inches (220 cm) per season, but snow depth is usually shallow. A strong warming sun due to the high elevation can quickly melt snow cover during the day, and Chinook winds bring rapid warm-ups throughout the winter months.

The Hotel Boulderado doesn't actively advertise its ghosts like some historic hotels, but there are reports that rooms 302 and 304 are haunted. People claim they see a woman wearing white in the hallways on the top floor, experience electrical disturbances, and hear ghostly voices. As the story goes, the supposed haunted rooms were the location of a double suicide.

The television sitcom Mork & Mindy (1978-1982) was set in Boulder, with 1619 Pine St. serving as the exterior shot of Mindy's home. The New York Deli, a restaurant in the Pearl Street Mall, was also featured prominently in the series.

A Colorado Daily poll found that seven out of 10 Boulderites own bicycles. In fact, bicycling is so highly regarded in Boulder that sometimes the city plows the Boulder Creek bike path before they plow the streets!