In what was a dour game, the solitary strike came in the 39th minute from former Aberdeen and Ross County striker Michele Lombardi. He showed great body strength to hold off a defender and get on the end of a through ball from Barry Sellars before tucking his shot past United keeper Mark McGeown.
Ayr ought to have taken the lead earlier when, just after three minutes, Raymond Logan powered in a header but Forfar keeper Sean Murdoch smothered the ball on the line.
At the other end Andy Jackson was denied a goal for Forfar by the legs of McGeown.
Jackson looked to have carved out the opener for Forfar in the 18th minute when he powered his way into the box and squared the ball to Paul Lunan, but his team-mate had strayed offside before finishing.
Quick thinking by Sellars saw him thump back a poor goal kick from 40 yards out but the ball ran just wide.
Ayr started the second half strongly, and Ian Cashmore flashed a header just past the upright.
Forfar went closest in the second period when Lunan should have doubled his side's lead in the 80th minute only to lob his shot weakly into the keeper's arms.