No portion of this state route is included within the National Highway System (NHS).

The NHS is a network of highways identified as being most important for the economy, mobility and defense of the nation.

It was originally routed along its current path between Granville, at the eastern intersection with Broadway Avenue, the former routing of SR 16, and the southern end of Mount Vernon.

That point marked the original northern terminus of SR 661.

[6][7] On the northern end, SR 661 was re-routed off of Granville Road onto Blackjack Road heading east to a new terminus at SR 13 in the southern end of Mount Vernon in 1983.