Its northern terminus is at a roundabout junction with SR 4 about 6 miles (9.7 km) southwest of the city limits of Sandusky.

Along its way, SR 99 runs through the western half of Huron County and the southwestern portion of Erie County.

No segment of SR 99 is included within the National Highway System, a network of highways identified as being most important for the nation's economy, mobility and defense.

[2][3] In 1937, the highway was extended north from Monroeville through its present northern terminus at SR 4, and then continuing northwest from there along Skadden Road, and north along Bardshar Road before coming to an end at its intersection with US 6 in the western end of Sandusky.

[5][6] By 1966, SR 99 was shortened to its present routing when the portions of the highway along Skadden Road and Bardshar Road north of SR 4 were removed from the state highway system, and jurisdiction of these roadways was turned over to Erie County.