Forward Benji Michel scored Orlando City's goal and it looked like it would hold up until Jahn's heroics in the second of five stoppage minutes.
Midfielder Jake Mulraney sent a left-footed cross from the left side toward the net and Jahn rose up and sent the header into the left corner to tie it up and earn Atlanta United a point.
Atlanta United is just 1-4-2 over their past seven regular-season matches.
Orlando City has two wins and two ties over its last four matches.
Orlando City allowed a limited number of fans to attend the game at Exploria Stadium despite the coronavirus pandemic. The attendance wasn't immediately announced.
Orlando City outshot Atlanta United 15-9 but placed just four shots on target. The Five Stripes put six on target.
Orlando snapped a scoreless tie with a goal in the first minute of first-half stoppage time.
Forward Chris Mueller took a corner kick from the left side and sent the ball toward the traffic in the front of the net. Forward Daryl Dike collected the ball and turned and shot and the ball went directly to Michel, who redirected it with his right foot past Guzan.
Atlanta United had a major chance to tie the score in the opening minute of the second half but forward Erick Torres sent a point-blank header wide to the left.
Orlando had an opportunity to add to its lead in the 80th minute when midfielder Nani took a free kick. His blast caromed off the left goalpost and midfielder Junior Urso's floating follow toward the net was chased down and cleared by Atlanta midfielder Emerson Hyndman.