The road network along the Snoqualmie River developed in the early 20th century to connect communities that had been established near a branch of the Chicago, Milwaukee, St. Paul and Pacific Railroad.

[5] SR 203 travels through Monroe on Lewis Street, passing through residential neighborhoods and the west side of Al Borlin Park before reaching the city's downtown.

[10] The Snoqualmie Valley was settled in the late 19th century and its communities were originally connected by a series of unpaved country roads, among the earliest to be built in King County.

[12] A branch of the Chicago, Milwaukee, St. Paul and Pacific Railroad was built along the east bank of the river in 1911, necessitating the relocation of the road and the entire town of Cherry Valley (now Duvall).

[13][14] The northernmost section of the road, along Lewis Street in downtown Monroe, was paved by the city government in 1912 and connected to a bridge over the Skykomish River that was rebuilt as a steel structure in 1915.

[17] By the mid-1920s, the paved Snoqualmie Valley highway extended south from Monroe to the King–Snohomish county line north of Duvall, while the remainder was described as an improved road with varying quality.

[30] A second study commissioned in 2003 determined that a highway built north of Snoqualmie would not be economically feasible and would fail to sufficiently benefit freight interests to justify its multibillion-dollar cost.

[31] In the 2000s, WSDOT began a series of construction projects on sections of the SR 203 corridor that aimed to improve traffic congestion and increase road safety.

A roundabout was constructed in southern Duvall to replace the unsignalized intersection at Northeast 124th Street in 2004, the first to be built on a rural highway in Western Washington, and received favorable feedback from local residents.

[33] A 2004 study of potential improvements to SR 203 also suggested the addition of a rumble strip in the road's centerline, along with hard landscaping to force motorists to slow down when entering populated areas.