In July 1858, gold was discovered along the South Platte River in Arapahoe County, Kansas Territory.

Weld County was thrust into the media spotlight on the evening of November 1, 1955, when United Airlines Flight 629, a Douglas DC-6B airliner flying from Denver to Portland, Oregon, exploded in midair and crashed, killing all 44 persons on board the plane and scattering bodies, wreckage and debris over a six-square-mile area of the county.

The subsequent investigation of the accident revealed that Denver resident John Gilbert Graham had secretly placed a time bomb composed of 25 sticks of dynamite in a suitcase belonging to his mother, who was a passenger on the airplane.

"Weld County voters said this is an option we shouldn't pursue and we won't pursue it," said Weld County Commissioner Sean Conway, "But we will continue to look at the problems of the urban and rural divide in this state.

"[14] In response to Gordon's comment, Colorado Governor Jared Polis said, "Hands off Weld County.

Weld County lies within the relatively flat eastern portion of Colorado; the northeastern portions of the county contain the extensive Pawnee National Grassland and the Pawnee Buttes, which jut 350 feet (110 m) above the surrounding terrain and are surrounded by many small canyons and outcroppings.

Along the western border, hilly areas indicate the presence of the foothills of the Rocky Mountains further west.

Other major roads include US 85 and US 34, which intersect near Greeley, and State Highway 14, which runs through Ault.

Weld County is Colorado's leading producer of cattle, grain and sugar beets, and is the richest agricultural county in the United States east of the Rocky Mountains, and the fourth richest overall nationally.

[25][26] Weld County is also an important area of oil and natural gas production in the Denver-Julesburg Basin.

Except for Lyndon Johnson's 1964 landslide win over Barry Goldwater, it has not voted for a Democratic presidential candidate since 1936.