Franco Escobar scored for Atlanta, which won for the fifth time in its past six matches to remain a point ahead of the New York Red Bulls in the race for the Eastern Conference title and the Supporters' Shield, which also includes home-field advantage throughout the playoffs.
Atlanta will close the regular season next Sunday at Toronto FC, the defending MLS Cup champions who have already been eliminated from playoff contention. The Red Bulls, who beat Philadelphia 1-0 Sunday, play host to last-place Orlando City SC next weekend.
Atlanta also set new Major League Soccer records with a total home attendance for the 2018 season of 901,033 -- including 71,812 on Sunday -- and a single-season average of 53,002.
Atlanta broke the records it set in its inaugural season a year ago, when they drew 819,404 and an average of 48,200.
Escobar opened the scoring in the ninth minute. A Josef Martinez shot deflected off a defender and to Escobar on the right wing. His left-footed, one-timer from about 15 yards beat Chicago goalkeeper Patrick McLain and found its way just inside the left post.
The Fire tied the score on Michael De Leeuw's goal in the 24th minute. A through ball from Brandt Bronico sprung De Leeuw and Djordje Mihailovic on a two-on-none break against goalkeeper Brad Guzan. Mihailovic's pass allowed De Leeuw to tap the ball into an empty net.
Atlanta regained the lead just two minutes later on a Chicago own goal. Atlanta's Chris McCann drove down the left wing and sent a low cross into the middle. As McLain dove to his left to corral the cross, defender Johan Kappelhof slid in front of him and inadvertently deflected the ball into his own net.
Martinez, who has already set an MLS single-season record with 30 goals, failed to tally for the fourth straight game. Martinez put two of his three shots on goal, but was thwarted on a 15-yarder in the first half, as the ball ricocheted off McLain's shoulder, and by a diving stop in the second half.
Atlanta midfielder Miguel Almiron, an MVP candidate with 12 goals and 14 assists, missed the match because of a hamstring injury.
Atlanta United 2-1 Chicago Fire: Supporters' Shield within reach
Franco Escobar's first goal for the club and a Chicago Fire own goal ensured Atlanta United's quest for the Supporters' Shield remains on track.