The first half was a pretty tame affair, although Jerome Vareille hit the crossbar for the visitors in the 11th minute. Then Andy Reid was only inches away with a shot from 20 yards which whistled past the upright.
The game only came to life in stoppage time at the end of the first period when Forfar took the lead and immediately surrendered it.
A cross from the left from Ricky Waddell was only half cleared, and Paul Lunan swept home a left-foot shot from 12 yards.
While the home fans were still celebrating, Ayr flooded forward from the restart and Vareille flipped on a header to Chris Robertson, who slipped the ball under keeper Neal Ferrie.
The second half saw Forfar start brightly and new signing Martin Cameron hit the crossbar from just inside the box.
But they fell behind in the 64th minute when Vareille hit a long-range effort which Ferrie could only parry and Robertson knocked home the rebound.
Forfar had a strong claim for a penalty waved aside with just five minutes left as Ayr held on for the win.