McMahon passed for 2 touchdowns, setting up one of them with a 16-yard run, and threw a key block on another score Sunday as the Bears handed the Buffalo Bills their first defeat, 24-3.
The Bears (4-1) have won 33 of the last 35 games McMahon has started.
"Give all the credit to Jim McMahon," Coach Mike Ditka said. "He took a lot of rips and shots. We had to attack and Jim made things happen.
"Maybe we caught (the Bills) on an off day."
McMahon completed 20 of 27 passes for 260 yards, and threw touchdowns on plays of 4 yards to Emory Moorehead and 63 yards to Ron Morris.
"I'm glad I didn't go in for the touchdown," McMahon said of his run to the 4-yard line, which set up his touchdown pass to Moorehead. "I get paid for throwing, not running the ball in."
He also threw the key block on linebacker Cornelius Bennett when Dennis Gentry ran 58 yards for a touchdown on a reverse.
"Those kind of blocks aren't hard," he said. "The guy is running into you and all you have to do is fall down."
The Bear defense held Buffalo to zero yards rushing and sacked quarterback Jim Kelly 6 times for 56 yards lost. Richard Dent had 2 of the sacks.
"We are now going into the proper direction," Dent said. "I'm never surprised at my performance. I always strive to do my best."
Dent was in on 4 other tackles and created havoc in the Bills' offense all day.
"Richard Dent just won too many battles," Buffalo Coach Marv Levy said. "They played a heck of a lot better than we did. We hurt ourselves with our penalties. But the Bears beat us, not the penalties."
Kelly completed 20 of 37 passes for 274 yards.
The Bills' only meaningful drive came late in the first quarter when Kelly completed 3 first-down passes, including 1 for 33 yards to Thurman Thomas to get to the Chicago 13.
But the Bills (4-1) had to settle for a 28-yard field goal by Scott Norwood. It was his 13th straight, extending his club record. However, that streak came to an end when Norwood missed on a 46-yard attempt in the closing seconds of the half.