The only other team who went into week seven unbeaten, the Minnesota Vikings, suffered their first loss of the season with a 31-29 loss to divisional rival Detroit.
The trip to Santa Clara was the biggest test of the season yet for the Chiefs and coach Andy Reid's players showed they mean business in their Super Bowl title defense with yet another performance mixing rock solid defense with the clever quarterback play of Patrick Mahomes.
Two rushing touchdowns from Kareem Hunt gave the Chiefs are 14-3 lead in the second quarter but the 49ers fought back when, after Mahomes threw an interception, San Francisco's Brock Purdy led a seven-play touchdown drive which ended with him running in the final yard.
But Purdy, who threw three interceptions, was picked off by Christian Roland-Wallace and Mahomes took full advantage on the resulting possession.
Mahomes scrambled brilliantly for 33 yards to the San Francisco 4-yard line and finished off the drive by powering up the middle to score himself and make it 21-12.
Purdy had a chance to narrow the deficit but his throw into the end zone, intended for Chris Conley, was intercepted by Jaden Hicks and once again Kansas City scored on the possession with Mecole Hardman racing in on an 18-yard sweep.
A late touchdown pass from Purdy to Bell came too late for any comeback by the 49ers, who slipped to 3-4 on the season.
Gibbs roars for Lions
Jahmyr Gibbs ran for two touchdowns and 116 yards on 15 carries for the Detroit Lions, who take over at the top of the NFC North and the conference, sharing a 5-1 record with Minnesota.
The Vikings were up 10-0 at the end of the first quarter after Aaron Jones ran in a 34-yard touchdown and Will Reichard made a 57-yard field goal.
But Detroit grabbed control of the game with three touchdowns in an explosive second quarter.
Gibbs rushed 45 yards to get the Lions on the scoreboard before quarterback Jared Goff connected with Amon-Ra St. Brown on a 35-yard touchdown pass.
Gibbs finished off a 72-yard drive with an 8-yard rush to give Detroit a 21-10 half-time lead.
The Vikings got back in the game as quarterback Sam Darnold found Justin Jefferson with a 25-yard pass into the end zone.
Goff responded, finding Kalif Raymond with a short left pass the receiver grabbed for a 21-yard score putting Detroit up 28-17.
The Vikings cut the Lions' lead to five with two Reichard field goals in the fourth quarter before Detroit running back David Montgomery fumbled and Minnesota's Ivan Pace recovered and raced 36 yards for the touchdown.
The Vikings failed in a two-point conversion attempt, leaving the Lions behind by a point, but Detroit took the win with Jake Bates' 44-yard field goal.
Minnesota had one last chance to save their unbeaten record but Trevor Nowaske sacked Darnold to end the contest.
The Green Bay Packers are third in the NFC North but far from out of the picture after a thrilling 24-22 win over visiting Houston.
Brandon McManus' 45-yard, walk-off field goal ensured the Packers moved to 5-2 after a game where quarterback Jordan Love threw for 220 yards and three touchdowns.
Texans quarterback C.J. Stroud managed just 86 yards on 10 of 21 passing with running back Joe Mixon doing the heavy lifting with two touchdowns and 115 yards on 25 carries.
The Washington Commanders (5-2) lost quarterback Jayden Daniels to a rib injury but back-up Marcus Mariota threw two touchdowns in a 40-7 crushing of the struggling Carolina Panthers (1-6).
Saquon Barkley returned to haunt his former club, the New York Giants, rushing for 176 yards and a touchdown as the visiting Philadelphia Eagles won 28-3.
Eagles quarterback Jalen Hurts ran in two touchdowns and threw another to A.J. Brown as the Eagles moved to 4-2.
Later on Sunday, the Pittsburgh Steelers host Aaron Rodgers and the New York Jets.
