SR 191's southern terminus is at U.S. Route 6 (US 6) approximately four miles (6.4 km) south of Stryker.

Between its southern terminus and Stryker, SR 191 runs parallel to, and to the east of, the Tiffin River.

Inside the village limits of West Unity, SR 191 is known as Main Street.

In 1937, the highway was extended north from Stryker to its current northern terminus at US 20A and US 127 in West Unity.

[5][6] Then, in 1962, the highway took on its present form when the portion of SR 191 south and east of the existing US 6 was removed from the state highway system, with the segment from the current southern terminus to Evansport renamed as Williams County Road 22-A, and the part from Evansport east that, for the most part, straddles the Defiance-Williams county line given the name of Defiance County Road 1.