Its northern terminus is also at a T-intersection, this time with SR 185 on the Darke–Miami County Line nearly 1.50 miles (2.41 km) north of Bradford.

There is no section of SR 721 that is included within the National Highway System (NHS).

The NHS is a network of highways deemed to be most important for the country's economy, mobility and defense.

Throughout its history, the route has maintained the same alignment in the general vicinity of the boundary between the counties of Darke and Miami.

[5] By 1940, the remaining gravel portion of the highway between Laura and US 36 was paved.