Thistle travelled to face the Swallows in the Tom McIntyre Memorial Cup section looking to get revenge for defeat in the league last month.
After a spell of sustained pressure and several corners the home side took the lead 10 minutes from the break when they finally broke down the Thistle defence with the scrappiest of goals with Cormack eventually prodding the ball into the net.
Just on the stroke of halftime only an excellent save from Anderson prevented Bissett from doubling his side’s advantage.
Thistle started the second half in impressive fashion and should have levelled the match less than a minute after the restart when Barnes put Harrow 1 on 1 with Segarius but the livewire striker failed to test the keeper with time to spare.
Harrow was to make amends just a minute later though when a Barnes throw was knocked on by Gorgon into the path of Harrow who slammed the ball into the roof of the net.
Thistle took the lead for the first time when Jukes left Low in his wake before releasing Harrow who showed fine composure to fire past the advancing ‘keeper.
St Monans pulled themselves back on level terms when Cormack provided the finish converting a low ball in from Bissett.
Thistle restored their advantage 12 minutes from time when Petrie netted.
Jukes clipped the ball over the keeper and into the net for a comprehensive and deserved victory.
Alba Mortgage Solutions Man of the Match – Kris Ireland
Thistle – Anderson, Carson, McArthur (Capt), Ross (Petrie 62), Ireland, Jukes, Thomson (McShane 82), Gorgon (Foulkes 75), Cameron (Moir 62), Barnes, Harrow.
Sub – C Murray.
Tomorrow (Saturday) Thistle are again on Tom McIntyre Cup duty when they travel to face Glenrothes Athletic.