By 1940, the route was extended south to the Virginia border and a small incomplete portion between Snow Hill and Salisbury was finished.

[2] From the border, the route passes through a mix of woodland and farmland with a few residences in southern Worcester County before reaching Stockton.

[3][4] Here, MD 12 turns north to follow Washington Street, crossing the Pocomoke River out of downtown Snow Hill on a drawbridge.

[1][2] The route turns northwest again before heading into a mix of residential areas and farm fields on the outskirts of the city of Salisbury.

Upon entering Salisbury again, the route becomes municipally maintained and continues past a mix of residences and businesses with some industrial establishments.

[1][7] The portion of MD 12 south of Snow Hill was part of an old post road from the 18th century that ran the length of the Delmarva Peninsula.

By 1910, the highway was paved within Stockton, between Snow Hill and Indiantown, and for about 2 miles (3.2 km) south from the city limit of Salisbury.

[13][14] By 1927, MD 12 had been assigned to the highway despite a 4-mile (6.4 km) gap remaining from just north of the Wicomico–Worcester county line south toward Snow Hill.

[16] In addition, MD 12's present northern terminus at Main Street, including the bridge across the South Prong of the Wicomico River, was constructed in 1928 and 1929.

[22][23] An interchange is planned between MD 12 and US 113 south of Snow Hill in order to improve safety and traffic flow as a follow-up to widening US 113; however, the project is currently on hold.

[25] The proposed cost needed to build this interchange is $24.2 million, which includes engineering, acquisition of land, and the actual construction.