Earlier in the half, the Terriers could have had another goal when Oliver Norwood's effort hit the post as Mark Robins's side threatened to run away with the game against a beleaguered Leeds outfit, who saw manager Brian McDermott sacked on Friday evening.
Norwood went close again just before half time, but forced Kenny to save low down as the ball look destined to nestle in the net.
Instead, Leeds got back on level terms in first-half stoppage time when full-back Sam Byram crossed the ball from McCormack to rifle the ball home, having been second best for much the opening 45 minutes.
Leeds then looked to put the game beyond the Terriers before they could find a response, but it was the visitors who suddenly looked the likelier to score with Nahki Wells, Paul Dixon and Ward twice going close to an equaliser.
But it was United who found the back of the net when Byram again linked up with McCormack, and the ex-Middlesbrough forward made no mistake from six yards out to bag the 50th goal of his Elland Road career.
He got his 51st Leeds goal, and his hat-trick, after latching onto Cameron Stewart's cross to put the ball beyond the grasp of Smithies.
The Terriers' misery was complete when Alex Mowatt netted his first Leeds goal with eight minutes remaining to make it 5-1 to the hosts.
The victory is the first time in eight games that Leeds have registered a win, while Huddersfield drop to 14th in the Championship.