The modern corridor that SR 18 follows was added to the state highway system in 1931 as a branch of State Road 5 that ran from the main highway in Auburn to State Road 1 and U.S. Route 99 (US 99) in Federal Way via Peasley Canyon. The branch was retained by State Road 5 when it was re-designated as PSH 5 as a new highway code was established in 1937. A branch of PSH 2 was designated in 1949, traveling southwest from North Bend, around Tiger Mountain and through Auburn before ending at a junction with PSH 1 and US 99 in Milton.