Kamil Zayatte had given the visitors the lead, looping a header over Paddy Kenny late in the first half.
But McCormack used his pace to create a chance out of nothing, rounding the defender and slotting the ball under Chris Kirkland to leave Leeds unbeaten so far, and give Wednesday their first point of the season.
In what was a feisty Yorkshire derby, the home side really struggled to break down the Owls, who offered a real threat from set-pieces.
And it was from a corner that Zayatte put Dave Jones' side in front, steering his header over Kenny, after Michail Antonio had nodded the ball in his direction.
After the break the introduction of Michael Tonge seemed to give Leeds added impetus, and added more creativity in midfield.
Luke Murphy curled a shot over the crossbar before McCormack lit-up Elland Road with a sublime solo goal.
The Scot ran on to Luke Varney's pass, skipped round a wrong-footed Zayatte and slid the ball under Kirkland from just inside the area.
Both managers made changes in an attempt to try and sneak a winner, with Matt Smith coming on for the home side to add more of a threat in the air.
But it was Antonio who came closest to snatching the three points for the Owls, with his thumping low shot well saved by Kenny.
The Leeds goalkeeper kept out another drive from Antonio in the closing stages and at the other end McCormack's stoppage-time free-kick crashed into Wednesday's defensive wall to ensure it ended all-square.