The Hardin County section, a state secondary highway, extends 7.981 miles (12.844 km) from U.S. Route 31W (US 31W) in Elizabethtown north to KY 313 on the southern edge of Fort Knox.

[3] The 3.058-mile-long (4.921 km) Bullitt County segment is a rural secondary highway that runs from a dead end on the eastern edge of Fort Knox east to KY 61 at Belmont.

[4] The Hardin County portion of KY 251 begins at an intersection with U.S. Route 31W (West Dixie Avenue) in Elizabethtown.

It curves to the north-northeast and meets its northern terminus, an intersection with KY 313 (Joe Prather Highway).

The road then travels east and northeast through the southern edge of the community of Belmont before reaching its eastern terminus at KY 61.