Route 15 begins in the town of Stonington, at the intersection of Main Street, School Street, and West Main Street. The route proceeds north out of Stonington and through Deer Isle en route to the mainland of Maine. Upon leaving the islands to the south, Route 15 meets Route 175 and forms an approximately 2-mile (3.2 km) concurrency with it. Route 15 then splits off 175 and then joins Route 176 farther to the north. The two routes continue into Blue Hill, where Route 15 turns northward, while 176 goes to the east. Continuing north, Route 15 meets Route 199 in North Penobscot. Upon reaching Orland, Route 15 meets U.S. Route 1 and Route 3, and joins the concurrency northbound. The road sees the ends of Routes 175 and 46 before reaching the town of Bucksport. Upon reaching the town, US-1 and Route 3 split off and Route 15 turns northward towards Bangor. Route 15 continues towards Bangor, straddling the east side of the Penobscot River, along with US-1 to the west. Route 15 enters Brewer and follows South Main Street to an interchange with Interstate 395, at Exit 4. Route 15 joins I-395 westbound and runs concurrent with the Interstate (also with U.S. Route 202 between Exits 3 and 2) to its terminus, where Route 15 then takes Exit 1B to join Interstate 95 northbound for approximately 3 miles (4.8 km). Route 15 leaves I-95 via Exit 185 and joins Broadway heading north out of the city. Route 15 meets Route 221 at Six Mile Falls before entering Corinth. The route intersects with Routes 11 and 43 in Dover-Foxcroft, forming a 2-mile (3.2 km) concurrency with Route 11. Route 15 continues to the northwest, intersecting with Routes 6 and 16, forming a lengthy concurrency. The three routes proceed due west through Guilford, intersecting Routes 23 and 150, and then turning to the north in Abbott Village, where Route 16 splits off to the west. Routes 6 and 15 remain multiplexed for the better part of about 60 miles (97 km), traveling north to Moosehead Lake, and turning west to the small town of Jackman. Routes 6 and 15 finally come to an intersection with U.S. Route 201, approximately 12 miles (19 km) from the Canadian border. Route 15 ends at this intersection, while Route 6 joins US-201 en route to the border.