It passes through the communities of Portland and Lombardy before entering Smiths Falls and Lanark County, where it intersects former Highway 29 and Highway 43 and turns north. The route crosses the Rideau Canal and proceeds towards Carleton Place alongside the Ottawa Valley Railway, bisecting the communities of Franktown, Beckwith and Black's Corners. On the southern outskirts of Carleton Place, Highway 15 ends at Highway 7, just west of where it becomes a freeway. At one time, Highway 15 travelled concurrently with Highway 7 into downtown Ottawa.