Route 20 was one of the original 1922 highways. Its eastern terminus was originally at
Huntsville at
Route 10 (now
U.S. Route 24). Its western terminus was originally in
Kansas City at
Route 1. In 1926, the section west of Waverly became part of U.S. Route 24 and the section from Waverly to Marshall became part of
U.S. Route 65. 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. In 1934/1935, the section from
Glasgow to Huntsville became part of
Route 3 when that route was extended. 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.