Most of the current route of SR 124 follows a portion of the Lewis and Clark Trail. The highway from Piper Canyon Road to Waitsburg follows a small section of the route that Lewis and Clark took from Fort Clatsop in May 1806 before splitting in Montana. SR 124 became a state highway during the 1964 highway renumbering, in which the Washington State Department of Transportation (WSDOT) replaced the old system (Primary and Secondary Highways) with a new system (State Routes), which is still in use today. In the Primary and Secondary system, SR 124 from US 12 to SR 125 (then US 410) was Secondary State Highway 3D and SR 124 from SR 125 to SR 126 was Secondary State Highway 3E.