Thistle travelled to Dalgety Bay to face The Swifts in the semi-final of the Glenfield Autocentre Cup following a midweek that had seen any outside chance of promotion ended when they went down 3-2 at home to Strathmiglo United.
Tulleth forced a good save from a free-kick with the ball falling to Fox whose effort was cleared off the line.
Thistle were awarded a penalty after Barnes was caught with a high boot in the box.
Harris stepped up and fired into the corner of the net despite Chalmers going the right way.
As time ran out for the hosts, they began to push forward which in turn left some space at the back for Thistle to exploit and in the 82nd minute a pinpoint pass from Tulleth sent Nicolson through with the youngster doing brilliantly to hold off a couple of challenges before showing great composure to fire past Chalmers.
The Swifts continued to push forward in numbers but the clock was against them with only a consolation goal with the last kick off the match seven minutes into stoppage to time to show for their efforts.
LM Joinery Man of the Match – Liam Carson
Thistle: Wortley, Carson, Leadbetter, Bell, Whatmore (Nicolson 58), Gray, Fox (McColm 89), Tulleth, Allan (MacCabe 87), Barnes (Walker 78), Harris
Thistle will face Bridge of Earn in the final on Sunday at Moorside Park, home of Dundonald Bluebell with a 2pm kick off.