McCormack was twice put through by captain Rodolph Austin to improve his goals tally to eight for the season and four in five games.
The teams were led out by Leeds legend Norman Hunter, who celebrated his 70th birthday earlier in the week and was the club's guest of honour.
The former England defender was still making his way to his seat when teenage midfielder Alex Mowatt forced an instinctive save from Yeovil goalkeeper Wayne Hennessey, but chances were few and far between in a first half of frustrating stalemate.
Shane Duffy, who this week had his loan spell from Everton extended, broke up several promising Leeds moves, but Matt Smith should have opened the scoring after 20 minutes when the Leeds striker swung his boot at fresh air a few yards out and Mowatt curled a left-footed shot tantalisingly close in the follow-up play.
Lowly Yeovil were failing to trouble Leeds goalkeeper Paddy Kenny, despite some promising approach play and their 3-1 win over Nottingham Forest last time out.
But with Duffy and Byron Webster in commanding form at the heart of the visitors' defence, Leeds struggled to create openings of their own.
Jamie McAllister and Sam Hoskins threatened to provide Kevin Dawson with the opening goal but the Dublin-born winger failed to make contact, with Leeds' defence in disarray.
Wing-back Stephen Warnock, who had been a doubtful starter, hobbled off four minutes before the interval to be replaced by Lithuanian Marius Zaliukas for his Leeds debut.
The home crowd were roused by a goal three minutes into the second half.
Austin made a storming run through the midfield and McCormack pulled the trigger from just outside the right corner of the six-yard box.
Leeds had some desperate defending to do as their opponents forced a series of corners, and home manager Brian McDermott responded by sending on Dexter Blackstock in place of Smith.
Austin almost snatched a second Leeds goal but his shot was deflected over the bar after another penetrating run, while at the other end Kenny had to stretch to turn McAllister's free-kick into the crowd.
The Austin-McCormack combination worked its magic again after 67 minutes when Austin's pass found McCormack whose 18-yard shot flew past Hennessey into the centre of the goal.
Yeovil made a double substitution, Paddy Madden and Kieffer Moore replacing Liam Davis and Kevin Dawson, but their neat passing had no end product as Leeds bounced back from their West Yorkshire derby defeat at Huddersfield.