A 69th minute penalty from Forfar's top scorer Paul Shields was all that separated the sides and the home team had to thank keeper Michael Brown for playing a huge part in their victory.
Brown pulled off a string of great stops to ensure his side were perfectly placed to take advantage of Sheilds' spot-kick.
Forfar started brightly and were on top for the first quarter of an hour with Ricky Waddell and David McClune going close.
However, Ayr worked their way into the game and Brown became the busiest player on the pitch as he denied Stuart Boyd and Paddy Connelly twice.
However, the visitors were dealt a blow five minutes from the interval when keeper Ludovic Roy had to limp off with a groin strain.
Either side of half time, Brown made brilliant finger tip saves before the deadlock was finally broken in the 69th minute.
A quick break caught Ayr napping and, as Waddel burst into the box, Richard Buckley brought him down with a desperate lunge.
Forfar's top scorer, Sheilds, made no mistake from the spot.
Moments later, Sheilds almost doubled Forfar's lead with a glancing header which caught the wrong side of the woodwork.
Right at the end, Darren Johnson made a fantastic triple stop to ensure his side pushed Forfar all the way.