It serves to connect the cities of Union, Gaffney and Shelby, North Carolina.

In Gaffney, it goes through the downtown area along Fredrick Street and overlaps with the Overmountain Victory National Historic Trail.

[4] In 1942, it was extended again along new primary routing to SC 56 south of Glenn Springs.

[6][7] In 1989, SC 18 was extended to its current western terminus at US  176/SC 215, replacing part of US 176 and all of US 176 Bus.

Two or three years later, its Westminster–Walhalla segment was eliminated, and it was routed from Westminster to the Georgia state line northwest of Long Creek.

South Carolina Highway 18 Alternate (SC 18 Alt.)

It was established in 1940 as a redesignation of the SC 18 mainline between Limestone Street and U.S. Route 29 (US 29).