The state highway departs the city of Delaware heading northeast into Brown Township amid a vastness of open farmland and the occasional home, passing underneath the CSX railway prior to passing T-intersections with Harris Road and Skinner Road. At the County Home Road intersection, SR 521 turns east, and enters the hamlet of Kilbourne, where it passes amid some homes and businesses, as well as intersects Old State Road. East of there, SR 521 passes through woodlands as it crosses over Alum Creek and passes its intersection with Hogback Road.
State Route 521 originates in Delaware City, heading north-east into Brown Township amidst open agricultural landscapes and isolated homes, passing beneath the CSX railway before intersecting T-intersections on Harris Road and Skinner Road. As it approaches Kilbourne, SR 521 turns east and passes through residential neighborhoods, also crossing Old State Road. East of there, SR 521 traverses woodland areas as it crosses Alum Creek and intersects with Hogback Road.