Kansas City scored two touchdowns in the second quarter and two second half field goals to pull out a 27-13 victory on Sunday before a crowd of 55,669 at the Edward Jones Dome.
"We made a lot of mistakes out there," Rams running back Steven Jackson said. "There's not a pretty way to put it. We just didn't play good football."
The Rams, playing their first home game in four weeks, fell to 6-8 on the season with two games remaining. The AFC West-leading Chiefs improved to 9-5, the team's best record since going 9-7 in 2006.
"They played better than we did," Rams coach Steve Spagnuolo said. "They won the game and we lost. What I just told the football team was, if I have this thing figured out correctly, basically for us our regular season is over and now its playoff mode for us. We've got to win the next game — that's the one we're focused on and to have anything to do going forward. So that's kind of where we're at.'
Rams rookie quarterback Sam Bradford completed 21 of 43 passes for 181 yards and two interceptions and a quarterback rating of 40.9. Jackson rushed for 67 yards and a touchdown on 19 carries and had 37 receiving yards on five catches. Danny Amendola caught seven passes for 60 yards.
Kansas City running back Jamaal Charles rushed for 126 yards and a touchdown on 11 carries, while Thomas Jones added 62 rushing yards on 22 carries. Quarterback Matt Cassel, making his return after an appendectomy, completed 15 of 29 passes for 184 yards, with one touchdown and one interception.
St. Louis had been plagued by rough starts the last three weeks, but that wasn't the case on Sunday as the Rams outscored the Chiefs 6-0 in the first quarter.
The Rams moved the ball well on their first series, covering 63 yards on 12 plays before back-to-back false start penalties caused the drive to stall and force the team to settle for a 37-yard field goal by Josh Brown.
"I thought we did a great job that first drive getting the ball down there and then once we got down there penalties just killed us," Bradford said. "It really seemed like after that drive, really up until the fourth quarter, it just seemed liked we struggled to find any sort of rhythm as an offense. Way too many three-and-outs. We've just got to be better as an offense."
A 42-yard punt return by Amendola put the Rams within striking distance on their next series, but after gaining two yards on three plays they again had to settle for a Brown field goal, this time from 52 yards out.
St. Louis got the ball back near midfield after Kevin Dockery intercepted a Matt Cassel pass and returned it to the Rams 48 yard line. But Spagnuolo's squad wouldn't be able to turn that possession into points, which became the beginning of a bad trend for the home team.
Dockery said the early lead was definitely the way they wanted to start.
"Then, from that point, we have to keep the storm coming," he said. "It seemed like from that point we couldn't really get anything going."
As the Rams offense sputtered, Cassel and the Chiefs got it going with back-to-back scoring drives in the second quarter that resulted in touchdowns.
Cassel connected with tight end Leonard Pope for a 2-yard touchdown on third down to give Kansas City a 7-6 lead with 6 minutes and 49 seconds left in the second quarter.
Then running back Jamaal Charles capped off a nine-play drive with a 2-yard touchdown run on the Chiefs' next possession, giving the visitors a 14-6 advantage with 1:47 remaining in the first half.
Kansas City got a 53-yard field goal from Ryan Succop on its first series of the second half to extend its lead to 17-6 and added a 38-yard field goal from Succop early in the fourth quarter to make it 20-6.
The Rams pieced together another scoring drive in the fourth quarter, covering 72 yards on 12 plays and capping the drive with a 5-yard touchdown run by Jackson to cut the deficit to 20-13.
But the Chiefs answered quickly. Charles broke loose for an 80-yard run on the first play of the next series and on the second play Jones burst through the middle of the defense for a 2-yard touchdown run to push the lead to 27-13.
"We started out a little shaky but we got our feet under us, we settled down and started playing football and doing things we knew we had to do to have a chance to win," Chiefs coach Todd Haley said.