[1][2][further explanation needed] PTH 18 begins in the Rural Municipality of Killarney-Turtle Mountain at the North Dakota Border, with the road continuing into Rolette County towards St. John and Rolla as North Dakota Highway 30 (ND 30).

The border crossing lies only 6.5 miles (10.5 km) west from Turtle Mountain.

The highway heads due north through farmland for a few kilometres to pass through Lena, where it has an intersection with PR 341.

It becomes concurrent (overlapped) with PTH 3 (part of the Boundary Commission Trail), and the two head north to cross the Long River into the town of Killarney, with PTH 3 (as well as the Boundary Commission Trail) splitting off and heading west along the banks of Killarney Lake while PTH 18 continues north through neighbourhoods and a business district as it bypasses downtown along its eastern side.

PTH 18 leaves Killarney to cross the Pembina River and have a junction with PR 253 before entering the Rural Municipality of Prairie Lakes.