Since the mid-20th century, areas of Snohomish County have developed into an aerospace manufacturing center, largely due to the presence of Boeing in Everett.

John Work of the Hudson's Bay Company recorded the name as "Sinnahmis" in 1824, while the Wilkes Expedition of 1841 used "Tuxpam" to describe the Snohomish River.

[4] Snohomish County was originally inhabited by several Coast Salish groups, predominantly settled along the western coastline and near the region's rivers.

[2] The region was first charted and named by European explorers in the late 18th century, beginning with Captain George Vancouver and his British expedition.

[2] The Treaty of Point Elliott was signed at present-day Mukilteo on January 22, 1855, marking the cession of Coast Salish territories in the Puget Sound lowlands.

The Tulalip Indian Reservation was established to house the remaining tribes, including the Snohomish, Snoqualmie, and Skykomish.

[8] The territorial legislature designated Mukilteo, the area's largest settlement, as the temporary county seat in January 1861.

A countywide general election on November 6, 1894, chose to relocate the county seat to Everett, amid controversy and allegations of illegal votes.

The early economy of Snohomish County relied on natural resources, namely timber and mining, alongside agriculture.

The region was connected by railroads at the end of the 19th century, which also created new towns that experienced major population booms as emigrants arrived from other parts of the United States.

The county was among the largest New Deal aid beneficiaries in Washington due to its troubled economy during the Great Depression; the Works Progress Administration built major projects around Snohomish County, while the Civilian Conservation Corps developed wilderness and recreational areas around several work camps.

[2] During World War II, the county had several shipyards and airplane factories established to supply the United States Armed Forces.

[2] A post-war population boom brought new suburban development to Snohomish County, where bedroom communities were built alongside new highways to Seattle.

[12][15] The county's surface is covered by plains and rolling hills in the west, where the majority of settlements are, and mountainous terrain in the east.

The mountain range includes the highest point in Snohomish County: Glacier Peak, at 10,541 feet (3,212.90 m) above sea level.

[17][18] The lowland areas of western Snohomish County generally has a temperate Mediterranean climate similar to the rest of the central Puget Sound region with dry summers and wet winters.

[16] The county's weather is heavily influenced by maritime systems, pushed by prevailing westerly winds but dampened by the Olympic Mountains.

[18] The Puget Sound Convergence Zone, a known meteorological phenomenon, runs through southwestern Snohomish County and causes narrow bands of precipitation.

[20] Approximately 68 percent of land in Snohomish County is classified as forestland, which is predominantly located in the eastern portions.

[32] The county government's powers and structure is defined by a charter that is updated every 10 years with amendments that are presented to voters for approval.

[39] Reardon resigned on May 31, 2013, amid a series of political scandals, and was replaced by former Snohomish County Sheriff and state legislator John Lovick for the remainder of his term.

[44] The District Court has nine judges that handle infractions, small claims, and domestic violence; it is divided into four geographical divisions.

[49][51] Snohomish County also had 68 private schools in 2018, of which many were affiliated with religious organizations or offered alternative education methods.

[60] The Seattle-based University of Washington (UW) had previously accepted a 2007 proposal from the state legislature to study and develop a branch campus in the county.

[61] UW selected finalist sites in Everett and Marysville, but halted further planning in 2008 due to the state government's inability to fund the $800 million project.

[60][62] Local officials approached WSU in 2011 with a proposal to gradually develop an Everett campus, which was authorized by the state legislature after UW backed out.

[66][67] Since 1975, Edmonds College has been the host of the Lynnwood branch of Central Washington University, which offers bachelor's degrees for transferring students.

[69] Residents receive much of their information from Seattle-based media, the most prominent of which include The Seattle Times and regional TV news stations.

[88][89] Intercity rail service is provided by Amtrak, which has two lines operating within Snohomish County: Amtrak Cascades between Seattle and Vancouver, British Columbia, stopping in Edmonds, Everett, and Stanwood station; and the Empire Builder between Seattle and Chicago, Illinois, stopping in Edmonds and Everett.

[93] The county also has several private airports, including the Frontier Airpark and Green Valley Airfield in Granite Falls.