US 1 bypassed the portion of the state road west of Octoraro Creek when the highway's modern route crossing the Susquehanna River at Conowingo Dam was completed in 1928. The old highway west of Octoraro Creek—which followed Old Conowingo Road, Ragan Road, and Connelly Road—became a western section of MD 273. This section was transferred from state to county maintenance in a road transfer agreement on May 8, 1958. The main section of MD 273 was extended west slightly when US 1 was relocated onto the Half Mile Turn to eliminate the right-angle turn at the intersection of Telegraph Road and Sylmar Road in 1935. US 1 was reconstructed through Cecil County in the early to mid-1950s, including the Harrisville–Sylmar portion of what is now MD 273. The U.S. Highway was reconstructed and widened from Conowingo through Harrisville to Rising Sun in 1952 and 1953. The highway from Rising Sun to Sylmar was reconstructed and widened in 1954 and 1955. MD 273 was extended west through Rising Sun and Harrisville to its present western terminus when US 1's Rising Sun Bypass opened in 1957.