Arniston were hit by a double whammy when they were reduced to ten men then lost a second crucial goal to the Jags just three minutes later.
Arniston could have taken an early lead with a deflected Kyle Doig shot forcing goalkeeper David Hay to concede a scrambled corner. A stunning overhead kick by Guy Kerr was also superbly tipped over the bar for another corner which was eventually cleared up by former Arnie player Tadg Moriarty.
Dalkeith survived that early pressure and then shocked their hosts by taking a 1-0 lead after only eight minutes played when a shot by Jonny Scott-Mercer was deflected into the Arnie net off Greg Callaghan for a fortuitous Thistle opener.
In the 12th minute a good effort by Arnie’s Adam Murray beat Hay but just cleared the far away post by inches.
This Midlothian derby attracted a good crowd despite the wild, wet and windy conditions which challenged the players, and in 22 minutes Arniston’s determination was rewarded with an equaliser. Lumbert Kateleza was the provider as Murray’s finish squared the match.
With better weather in the second half, play swung from end to end in a mainly midfield battle with few goalmouth incidents testing either keeper, except when Guy Kerr had the ball in the Thistle net only to be ruled offside. Darren McTiernan’s miskicked chance in 58 minutes kept the scores even.
But the stalemate changed in the 68th minute when, in a Thistle counter-attack, Greg Pullen pulled the jersey of the racing O’Reilly and, as last man he was red carded, leaving Arniston with ten men.
Arniston’s shocks continued just three minutes later when Dalkeith completed a crucial double blow for the hosts.
Just after substitutes Darren Leslie had come on for Arnie and Connor Spowart for Thistle, the visitors went 2-1 up when a low cross by Scott-Mercer was headed home by Mikey Hunter to further rattle the Arnie.
Sean Lally could have added a third for Thistle in 75 minutes but he miscued his shot close in before goalscorer Hunter was booked for a foul on Kateleza in the Arnie half.
Even so Arnie might have snatched a late equaliser but Darren Leslie sent his parting header wide.