The highway's eastern terminus is southwest of Joplin, Missouri, near Loma Linda, at an interchange with Interstate 44, with which it shares with US 166.

US 400 and US 54 begin a long concurrency at Mullinville which passes through Greensburg, Pratt and Kingman before entering Wichita.

US 400 continues east through several small towns before turning southeast to go through the Fredonia area and intersecting US 75 at Neodesha.

[6] In a May 3, 1995 resolution, it was approved to move US-400 and K-96 to a new alignment between northwest of Fredonia and Neodesha, at that time the overlap with K-47 was removed.

[citation needed] Construction on the first section of the East Kellogg improvement project started in August 2015.

Construction on a second project began in 2016, to continue widening the highway to a six-lane freeway between Greenwich Road and K-96.

[11] On April 16, 2020, vandals damaged an estimated $50,000 worth of construction equipment, which included a bulldozer, excavator and an off-road vehicle.