Brechin opened the scoring after 11 minutes when a Grant Johnson pass put Steven Hampshire free on the right, and his cross was fired home on the near post by Chris Geddes.
Forfar were out of luck two minutes later when Darren Gribben shot against the bar and Paul Tosh couldn't force home the rebound.
Brechin made it 2-0 after 36 minutes when a Stuart Callaghan cross was handled by Gary Lynn and Callaghan stepped up to score from the spot.
Forfar got a goal back three minutes from half time. John Ward was judged to have fouled Gribben, and Gribben made no mistake from the spot.
Forfar were out of luck after 58 minutes when Paul Lunan headed against the bar following a corner. Brechin went straight upfield and stretched their lead to 3-1 when Hampshire cut inside and lashed home a vicious drive.
Bechin should have gone further ahead after 68 minutes when Geddes crossed for substitute Iain Russell to hit the bar from close range.
Forfar got back to within a goal in the 84th minute when Tosh powered home a header from a Kevin Webster corner.
Brechin made sure of the points in the final minute. A shot from Kevin Byers was parried by Murdoch and Geddes was on hand to score his second.