Monroe was originally founded in 1864 as the town of Park Place, located at the river confluence among several existing settlements in the Tualco Valley.

It is home to the Monroe Correctional Complex, which absorbed the original reformatory in 1998, and the Evergreen State Fair, which runs annually in late summer.

[5] The confluence itself was known as Tualco (Lushootseed: squa'lxo), and a nearby Skykomish village named S'dodohobc acted as a trade post between several Coast Salish groups.

[8] The Skykomish were among the tribes to sign the Treaty of Point Elliott in 1855, effectively ceding their traditional territories, including the Tualco and confluence areas.

[6] The area around modern-day Monroe was surveyed by George B. McClellan and the U.S. Army Corps of Engineers during their expedition to find a suitable pass for a railroad across the Cascade Mountains.

[8] The Treaty of Point Elliott was not fully ratified until 1859, but the first American settlers had already arrived and claimed squatters rights to homestead in the Skykomish Valley.

[6][9] Two other settlers arrived in 1860: Salem Woods, who claimed a small prairie to the northeast of Tualco and was later elected county sheriff; and Charles Harriman, a territorial legislator who settled in Park Place.

[14] The Great Northern Railway chose a route over Stevens Pass in the late 1880s for its transcontinental railroad connecting Seattle to St. Paul, Minnesota, bringing new development to the Skykomish Valley.

[15] In late 1892 and early 1893, several merchants in Park Place moved their buildings to the south side of Tye City using teams of oxen, horses, and a steam thresher.

[11] The completion of the railroad attracted lumber operations to the Monroe area, boosted by the opening of the first shingle mill in 1894 and the first sawmill on Woods Creek in 1897.

By the end of the decade, Monroe had also gained a new school building, telephone service, a local newspaper, a full-time doctor, and paved sidewalks.

[18] On the morning of September 16, 1901, a fire started at the Odd Fellows community hall and spread to nearby buildings, destroying the only complete block of businesses in Monroe.

[32][33] Within a year, the plant was producing 250,000 pounds (110,000 kg) of condensed milk per day;[34] the Carnation condensery was later closed in 1928 and destroyed in a fire on March 23, 1944.

[35] A new train depot was constructed by the Great Northern Railway in 1909 to serve the mainline as well as a branch line traversing the Snoqualmie Valley that was opened two years later by the Milwaukee Road.

[36][37] The local timber industry declined and was replaced by a larger reliance on agriculture, namely dairy, vegetable, and berry farms on the logged-off lands around Monroe.

[41] The county government opened a new 100-bed hospital on the poor farm complex in 1925 at a cost of $92,000 (equivalent to $1.27 million in 2023 dollars)[20] to provide services to local residents.

[43] At the onset of the Great Depression in 1929, several lettuce farms in the Monroe area had folded and been acquired by the Frye Company, which provided employment through the decade for 1,000 residents.

[44] Service organizations in the town ran charity assistance programs for unemployed residents and their families, raising money from large employers to fund food and clothing donations.

[45] The Works Progress Administration (WPA) began civic improvement projects around Monroe in 1933, including repairs to damage caused by a major flood in February 1932.

[6] By 1949, the local government had twice rejected proposals to become a third-class city because of the increased operating costs needed; it also lacked a full-time fire department.

[11] A major earthquake struck the Puget Sound region on April 29, 1965, causing severe damage to the original Monroe High School and its annex.

[54] The easier car access made Monroe into a bedroom community for Everett, Seattle, and the Eastside region, with new suburban subdivisions being built around the city and annexed by the end of the decade.

[56] The former Frye lettuce farm in western Monroe was sold in the late 1980s to an Eastside-based real estate developer, which proposed the "Fryelands" residential and industrial neighborhood.

[70] The city limits are generally defined by Lake Tye and Fryelands Boulevard to the west, the Skykomish River to the south, Woods Creek to the east, and to the north by Milwaukee Hill and other foothills.

[69][73] The south and east edges of the city along the Skykomish River are within a 100-year flood hazard zone and also include habitats for migratory birds and game animals.

[75] Monroe also sits on a local fault line that produced a pair of minor earthquakes on July 12, 2019, that were felt as far as Seattle and Vancouver without causing damage.

[76] Monroe has a general climate similar to most of the Puget Sound lowlands, with dry summers and mild, rainy winters moderated by a marine influence from the Pacific Ocean.

[125][127] The pilot for the web television series The Man in the High Castle features a scene filmed at the Lewis Street Bridge on State Route 203.

[190][191] The Sky Valley Educational Center was the subject of several lawsuits filed by 200 students, parents, and teachers against Monsanto over exposure to toxic chemicals, including polychlorinated biphenyls, at the building.

[198][199] The city is bisected by the Scenic Subdivision, a major railroad owned by BNSF Railway that is used for freight and Amtrak's Empire Builder passenger service.