SSH 1S was created in 1937 during the formation of the primary and secondary state highways, traveling on a route east from Primary State Highway 1 (PSH 1) and U.S. Route 99 (US 99) north of Mount Vista to intersect SSH 1U in Battle Ground and turned north towards Amboy and west to end at PSH 1 and US 99 in Woodland. After PSH 1 and US 99 was realigned onto a four-lane limited-access highway in the late 1940s, SSH 1S was extended south over the old highway to intersect PSH 1 and US 99.