SR 173 officially begins at the Lincolnville Ferry Terminal of the Islesboro Ferry (connecting to the town of
Islesboro). The route winds through the community of Lincolnville within the town of the same name before reaching
U.S. Route 1 (US 1). Now following Beach Road, SR 173 generally heads northwest through forested area paralleling Frohock Brook at first as it curves around hills of
Camden Hills State Park. The road makes sharper curves at the communities of Carvers Corner and Stevens Corner until it reaches Drakes Corner where
SR 52 forms a
concurrency with SR 173. The two routes together head into Lincolnville Center, the home of the administrative buildings of the town, on Main Street. The road intersects
SR 235 at its northern terminus before SR 52 splits from the concurrency allowing SR 173 to continue alone on Searsmont Road. It enters the town of
Searsmont and curves around Levenseller Mountain. At Bickfords Corner, SR 173 makes a sharp turn to the southwest to cross a small stream then travels west. The road comes to a
T-intersection with
SR 131 where SR 173 heads southwest on Main Street with SR 131 to downtown Searsmont. Just south of the
St. George River, SR 173 ends its concurrency with SR 131 and heads west along Woodmans Hill Road through a wooded area. It later curves to the northwest and passes through the unincorporated communities of Woodmans Hill and
South Montville (the latter of which is along SR 173's short portion within
Montville). Along Stevens Pond Road, the route reaches Trues Pond Road and heads west through downtown Liberty as the town's main street. The route ends at an intersection with SR 220. ==Junction list==