Its eastern terminus is at SR 656 in the hamlet of Olive Green, located approximately five miles (8.0 km) north of Sunbury.

[2][3][4] The aforementioned signalized intersection with US 36/SR 37 on the east side of Delaware marks the start of the signed portion of SR 521.

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.

Resuming an easterly trek, after passing the unusually-named 3 Bs & K Road, SR 521 crosses into Kingston Township.

Continuing east through farm country and passing the occasional house, State Route 521 passes through intersections with Carter's Corners Road and Blue Church Road, then turns to the east-southeast prior to entering Porter Township and arriving at its junction with SR 61.