DE 9 was first designated by 1936 to run from U.S. Route 113 (US 113, now DE 1) southeast of Dover north to US 13 in Smyrna, following its current alignment to Leipsic and Smyrna-Leipsic Road to Smyrna. In the 1950s, the route was extended to US 13 Alt. (now US 13) in Wilmington, in which it was rerouted at Leipsic to follow its current alignment to Wilmington. DE 9 was extended further north to DE 2 by the 1970s. The route was rerouted to bypass downtown New Castle in the 1980s. The intersection with DE 1 was reconstructed into an interchange in 2009.