Hell Gate Town, the county seat, was at the confluence of the Clark Fork and Bitterroot Rivers.

The area encompassing today's Missoula County became part of the United States as a result of Oregon Treaty of June 14, 1846.

It was part of the Oregon Territory's Clark County, which replaced the District of Vancouver September 3, 1844.

The county made up the region between modern-day Idaho and the Continental Divide north of the 46th parallel.

Local wildlife includes white-tailed deer, black bears, osprey, and bald eagles.

During the winter months, rapid snow melt on Mount Jumbo due to its steep slope leaves grass available for grazing elk and mule deer.

[12] Native riparian plant life includes sandbar willows and cottonwoods, and Montana's state tree, the ponderosa pine.

Notable ones include Dalmatian toadflax, spotted knapweed, leafy spurge, St. John's wort, and sulfur cinquefoil.

[13] The Norway maples that line many of Missoula's older streets have also been declared an invasive species.

However, the mildness is also induced by the dampness, as unlike much of the rest of the state, precipitation is not at a strong minimum during winter.

However, temperature differences between day and night are large during this time and from April to October, due to the relative aridity.

The commission has authority over all legislative, executive, and administrative issues throughout the county not specifically reserved by law or ordinance to other elected officials.