Craig Johnston’s goal earned Montrose a 1-0 play-off advantage and ended Airdrie’s 20-match unbeaten league run.
Johnston fired home in the ninth minute of the cinch Championship play-off semi-final first leg at Links Park after the Diamonds failed to deal with an Andrew Steeves cross.
Lyall Cameron and Scott Agnew came close from long range at either end and the former forced a good save from Airdrie goalkeeper Max Currie on the counter-attack.
Montrose could have added to their lead after the break as Michael Gardyne curled just wide and Currie twice saved from Johnston while Dylan Easton came close for the visitors as they pressed for an equaliser.
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