Its western terminus was originally in Kansas City at Route 1.

In 1933/1934, Route 20 was extended west on its current alignment to Higginsville.

This section had been planned as Route 96 (Odessa to Marshall) in 1922, but was soon dropped from the state highway system.

The next year, the section from Marshall to Glasgow became part of Route 240.

Route 20 connects the towns of Higginsville, Corder, Alma, Blackburn and Marshall.