Before SR 9 was created, several other roads used the route of the current highway. The first was a roadway extending from the current southern terminus to Snohomish established by 1895 and another road between Arlington and Sedro-Woolley by 1911. The current SR 542 concurrency was first established in 1925, when a branch of State Road 1 from Bellingham to Mount Baker was added to the state highway system. These roads were combined and several other roads were added to create Secondary State Highway 1A (SSH 1A), which originally ran from Woodinville to Blaine in 1937. A branch of SSH 1A connected the mainline to the Canada-US border in Sumas, but was later included into SSH 1A when the Blaine to Sumas segment was deleted in 1953. A highway renumbering in 1964 introduced the sign routes that would be co-signed with the existing system until 1970, one of which would replace SSH 1A, SR 9. SSH 1A / SR 9 extended south to Woodinville until 1965, when it was shortened to SR 202, later SR 522, which wasn't complete yet. SR 9 was not complete between Lake Stevens and Arlington until after 1966.