The state highway runs 18.50 miles (29.77 km) from U.S. Route 50 (US 50) in Easton north to MD 213 south of Centreville.

The state highway heads east as two-lane undivided Black Dog Alley for a short distance before turning northeast onto Cordova Road and leaving the town of Easton.

MD 309 heads north through a mix of farmland and forest parallel to an abandoned railroad grade owned by the Maryland Department of Transportation to the east of the road.

After passing through the village of Cordova, the state highway reaches a junction with the southern terminus of MD 303 (Tappers Corner Road).

North of Queen Anne, the state highway traverses farmland, meets the southern end of MD 481 (Damsontown Road), and passes through the hamlet of Starr before reaching its northern terminus at MD 213 (Centreville Road) south of the town of Centreville.