The highway connects US 29/SR 14 in West Point, just east of the Alabama state line, with US 80/SR 19/SR 96 in Jeffersonville, via Pine Mountain, Greenville, Zebulon, Barnesville, Forsyth, Gray, and Gordon.

Turning east and crossing into Pike County, the route again turns northeast and travels through Molena, where its concurrency with SR 109 ends, into Concord, where it curves east and travels to Zebulon.

Curving once again to the southeast, SR 18 crosses into Lamar County and travels to and around Barnesville, where it begins a concurrency with US 41/SR 7, before heading into Forsyth in Monroe County, ending its concurrency with US 41, and where it crosses I-75 shortly after leaving Forsyth.

The highway continues east into Jones County and enters Gray, where it is briefly concurrent with US 129, SR 11, and SR 22, and where it turns southeast once more in the direction of Gordon in Wilkinson County.

In Gordon, the highway turns south and southeast, crosses into Twiggs County, and reaches its eastern terminus in Jeffersonville.

SR 18 was established at least as early as 1919 on its current path, from Greenville to Barnesville.

[15][16] Between September 1938 and July 1939, the path of SR 19 from Barnesville to just southeast of Forsyth was truncated to the latter point.

[17][18] In 1942, two segments had a completed hard surface: a portion west of Chipley and from Woodbury to the approximate location of Molena.