Inverness and Kilmarnock played a tight game on Wednesday afternoon in driving wind and rain but it was Inverness who took their chances to secure the three points at the Tulloch Caledonian Stadium.
Kilmarnock started the brighter of the two teams when their left back found room on the left hand side and cut in but his curling effort didn't trouble Mackay in the Inverness goal.
Inverness had their first effort minutes later when Calum Ferguson was fouled just outside the box but Ryan Baptie's effort never troubled the Kilmarnock number one as it his shot sailed wide.
But it was the hosts who went ahead just sixteen minutes in. Polworth played a clever ball through ball which found Chris Blackett who for a minute lost his footing but regained his balance to slot it under the Kilmarnock keeper.
But Kilmarnock always looked dangerous going forward trying to take advantage of the young full backs. Most attacks started on the wings and Kilmarnock's number Dean Hawkshaw unleashed a drive from 25 yards but his shot just went wide.
Moments later Kilmarnock's centre forward turned on the edge of the box but his low drive could not beat Mackay in the Inverness goal.
But it was Inverness who took their chance when it came to them. Liam Polworth's surging run down the right-hand side beat the Killie left back to pass it into Ali Sutherland who laid it off to Calum Ferguson who slotted home his 10th of the campaign.
Kilmarnock had another couple of chances before the break with number Hawkshaw and Cairney both having low shots gathered by Mackay.