KY 420 begins at an intersection with U.S. Route 127 (US 127) south of Frankfort, within the southern part of Franklin County, where the roadway continues as Old Lawrenceburg Road.

The highway very briefly travels along the southeastern edge of the city limits of Frankfort.

It curves to the north-northeast and travels under a parking garage for the Kentucky State Capitol.

The two highways cross over the Kentucky River on the War Mothers Memorial Bridge and immediately split.

Northbound traffic then passes the Frankfort State Office Building, which houses the Kentucky Department of Revenue, and then intersects the southern terminus of KY 2261 (Holmes Street).

Immediately, the highway meets its northern terminus, an intersection with US 127/US 421 (Wilkinson Boulevard / West Frankfort Connector).