SR 99 starts in Leupp on the Navajo Nation and heads south to Interstate 40 west of Winslow. From there, the route runs concurrently with I-40, then exiting I-40 in Winslow onto Business Spur 40. In downtown Winslow, SR 99 heads south on SR 87 before separating from it outside of the city. The route heads towards the edge of the Apache Forest, where state maintenance ends.