The route runs through rural areas of Sussex County and passes through the towns of Seaford, Millsboro, and Dagsboro.

DE 20 begins at the Maryland border in Reliance, where the road continues west into that state as MD 392.

From the state line, the route heads east on two-lane undivided Stein Highway through farmland with some woods and homes.

The road crosses Williams Pond along Clear Brook before passing through commercial areas and coming to a bridge over the Nanticoke River.

Near the town of Blades, DE 20 splits from US 13 by turning east on two-lane undivided Concord Road.

[3][4] DE 20 turns southeast to form a concurrency with US 113 on four-lane divided Dupont Boulevard, entering Millsboro.

The two routes run past more businesses and a few homes, crossing Iron Branch before leaving Millsboro.

The road heads to the north of Indian River High School and runs through agricultural areas with some woods and homes.

Following this intersection, the route becomes Zion Church Road and runs through more rural land, crossing DE 54 Alt.

[7] What would become DE 20 was originally an unimproved county road in 1920, with the portion between Seaford and Concord under contract as a state highway.

[12] DE 20 was designated to run from the Maryland border in Reliance east to US 113 in Millsboro by 1936, following its current alignment to Seaford before heading south along what was then US 13 (Front Street) to Blades, where it turned east onto High Street and picked up its current alignment to Millsboro.