The highway is a west-to-east route that travels within portions of Fulton, Fayette, Clayton, Henry, Rockdale, Newton, and Walton counties that connects Fairburn and Monroe, while bypassing Atlanta.

It continues southeasterly to an interchange with Interstate 85 (I-85; James D. "Jim" McGee Memorial Highway) in Union City.

Farther to the southeast is SR 279 (Old National Highway), just prior to reaching the tripoint that is the meeting point of Fulton, Fayette, and Clayton counties.

The two routes run concurrently to the north and northeast through town, with an interchange with I-20/US 278/SR 12 (Purple Heart Highway), until SR 20 heads northwest on Sigman Road NE and SR 138 continues to the northeast in the northeastern part of town.

The roadway continues northeast for a mile as Charlotte Rowell Boulevard to Georgia State Route 11 in Monroe.

[2] An old portion of SR 138 with a steel truss bridge crossing the Alcovy River in Walton County still exists on privately owned property.

Some street signs on the northern bypass, now the SR 138 mainline, still incorrectly refer to the road as "Spur 138".