United ran up five goals but could have had many more against the relegation favourites on a great afternoon for the home side in front of goal.
They had to be patient after a slow start finally broke the deadlock ten minutes before the interval when Ryan Stevenson, making his home debut, lashed home a cutback from Gareth Wardlaw.
Ayr never looked back and doubled their lead just four minutes later when Stevenson turned provider to cross for Chris Robertson, who slammed an unstoppable shot into the top corner of the net from 12 yards.
Ayr turned on the style in the second half and were quickly 3-0 ahead when Frenchman Jerome Vareille headed home after Robertson had knocked down a Chris Strain free-kick.
Forfar were on the ropes and were soon four behind when Strain set up another goal this time for Wardlaw, as he glanced the header home from 12 yards.
Ayr streaked 5-0 ahead with only an hour played when Robertson stormed to the byline and cut back for Strain to fire into the roof of the net, as Forfar looked like losing by a cricket score.
However, Ayr slowed down and rarely broke sweat in the final half hour although Forfar required a superb last-ditch clearance on the goal-line from Craig Lumsden to deny Wardlaw a sixth goal late on.