Route 9L was assigned in the 1930s along the route of an old military road produced during the revolution. The route was given its designation because it is a spur route off of US 9, which is present at both its termini. The road was designated later as the Central Adirondack Trail by the United States Department of Transportation.
The highway starts at an intersection with US 9 and NY 32 in downtown Glens Falls. The road begins by intersecting with local roads in the city. East of the city limits in the town of Queensbury, Route 9L intersects Dix Avenue, a connector leading to NY 254. Route 9L turns to the northeast and intersects Route 254 itself east of its junction with Dix. Route 9L continues northward through Queensbury, passing Floyd Bennett Memorial Airport and running parallel to the Washington County line on its way toward and eventually across the Blue Line into Adirondack Park. NY 9L intersects NY 149, an east–west arterial connecting Interstate 87 to Vermont, just north of the park limits. The route continues into Brayton, a hamlet situated in northeast Queensbury. At a junction northeast of Glens Falls and north of Brayton, NY 9L meets Pilot Knob Road, a local roadway serving several communities located on the shores of Lake George and along the base of Pilot Knob.
Route 9L turns to the southeast, roughly paralleling the southern shore of Lake George. A mile west of Brayton, NY 9L intersects with Cleverdale Road, a road serving a small peninsula and the hamlet of Cleverdale. Just afterwards, it passes an open area called Joshua's Rock, with a cluster of buildings. This is Owl's Nest, the home of early realist novelist Edward Eggleston, a designated National Historic Landmark.
After passing the community of Rockhurst, the route intersects Assembly Point Road, another local road serving a minor peninsula and Assembly Point on its tip. Past this point, Route 9L tightly parallels the southern shore of Lake George southwest to the Town of Lake George, situated at the southwestern extent of the lake. The route comes to an end at a junction with US 9 and NY 9N south of the village of Lake George.
Route 9L had its bridge replaced starting in 2007 over the Halfway Creek in Queensbury, New York. The bridge, which was built in 1923, about a decade before 9L's commissioning, was just from the intersection with Haviland Road (Warren County Route 17). Construction on a new, larger bridge cost $1.3 mllion and is expected to be completed in October 2008.