CR 521 follows portions of the Military Road, a roadway built during the French and Indian War connecting Elizabethtown, New Jersey (now Elizabeth) with a string of fortifications constructed along the Delaware River in modern Sussex and Warren Counties in northwestern New Jersey.

The route reaches an interchange with I-80 and winds north through more rural areas, coming to a junction with CR 608.

The route enters wooded residential areas before coming into Hardwick Township at the CR 601 intersection.

In Hardwick Township, the road heads through a mix of farms, woods, and homes, crossing CR 659 and passing near White Lake.

The road passes between Stokes State Forest and the residential western shore of Lake Owassa before coming to a junction with US 206 and CR 521 turns north onto that route.

After this intersection, the road leaves the state forest and continues through wooded areas with some commercial establishments.

[4] In 1987, the New Jersey Department of Transportation did a study on the CR 521 bridge over the former Lackawanna Cut-Off, a two-lane structure in Blairstown.

The aging construction of the cut-off affected multiple structures in Warren and Sussex counties.

[5] Warren County residents asked if there was any progress in May 1998 on a new bridge over the Cut-Off, which NJ Transit considered for new passenger service.

[6] In March 2003, the Department of Transportation added an upgraded CR 521 bridge to their Capital Plan for 2004 construction.