Ayr finished with 11 men for the third week in a row as Brechin grabbed the points.
But top scorer Stewart Kean was wrongly red carded after a decision which left both sets of players shaking their heads.
After a poor first half, the game burst into life three minutes after the break.
Chris Templeman fired in a shot from the left which was deflected into the path of Paul Ritchie and he stabbed home from 12 yards.
The referee then lost the plot after 72 minutes when red carding Stewart Kean for an innocent challenge on Brechin keeper Craig Nelson.
Both a goal and a man down, Ayr had no chance of fighting back and the game turned nasty as the clock ticked down.
Brechin lost a man of their own when substitue Kevin McLeish - on the pitch for only 15 seconds - saw straight red for a sickening injury time tackle on Stuart McGrady.