Its current route takes it through the cities of Lima, Kenton, Marion, Galion, and Ontario; as well as the villages of Elida and Caledonia.

[1][2] The eastbound direction of SR 309 is signed south along Jameson Street to SR 117, then east along it while the westbound direction is signed along Central Avenue and Front Street.

[5][6] The original western split of US 30S and US 30N was in downtown Delphos at the current intersection of Main Street and Fifth Street, while the eastern split was in eastern Mansfield at the current intersection of US 42 (Ashland Road) and Park Avenue East.

The original western terminus was where Elida Road (SR 309) met East Fifth Street and Lincoln Highway (US 30).

Only two weeks after the official route was announced in September 1913, it had already been redefined to a straighter path between Lima and Galion; the rest of the straightening came later.

[14] There is a $4.1 million project in eastern Allen County to replace the current intersections at Thayer and Napoleon roads with roundabouts.