State Route 139 is a north–south road that stretches from New Boston, Ohio at its southern terminus to Jackson, Ohio at its northern terminus. This route offers a quick, yet narrow and sharp route to Jackson. U.S. Route 23 and State Route 32 offer a more straight route with a higher speed limit, though less direct.

State Route 139 begins at an intersection with U.S. Route 52 in New Boston. The route goes north through a couple of traffic lights before passing through Portsmouth, Eden Park, Twin Valley, Rubyville, and Clarktown before it continues due east toward Minford. From there it crosses State Route 335 and then begins to turn north once again. After reaching the Scioto County and Jackson County line, State Route 279 splits off of State Route 139 and heads toward Oak Hill. State Route 139 crosses State Route 32 just outside of Jackson and State Route 776 within less than mile of downtown. The route then briefly overlaps State Route 93 in downtown Jackson, terminating 0.15 miles (0.24 km) north where State Route 93 turns east.