The contest was important for the Chiefs, who are searching for a quarterback if Matt Cassel cannot open the season.
First-year head coach Todd Haley called the plays Thursday after offensive coordinator Chan Gailey was fired during the week.
Cassel is suffering from knee and ankle injuries and could be out a month. That will leave things in the hands of Brodie Croyle, who was 5-of-16 for 85 yards against the Rams, or Tyler Thigpen, who was 2-of-7 for 49 yards with two interceptions.
Kansas City had a 9-7 lead late in the third period when Thigpen's pass from his own 24-yard line was intercepted by Butler and returned 28 yards for a score.
Josh Brown added a 37-yard field goal in the fourth quarter and St. Louis held the Chiefs scoreless in the second half.
St. Louis starting quarterback Kyle Boller played a series, completing 3-of-4 passes for 24 yards.
The Rams completed their exhibition season at 3-1 and will start their regular campaign at Seattle. Kansas City (0-4) opens at Baltimore.