Leeds were out of sight by half-time as they found the net three times, firstly through McCormack after 18 minutes.
The Scotland international, who missed last week's World Cup qualifying win over Croatia, then played a major part in the next two goals from Rodolph Austin and Matt Smith. Smith added a fourth in the 74th minute to ensure his side's 4-0 win.
McCormack was twice the victim of desperate saves by Birmingham goalkeeper Darren Randolph before making it third time lucky in the 18th minute.
Randolph rushed out of his goalmouth but his attempted clearance cannoned off Leeds captain Austin to McCormack who shot into the unguarded net from 20 yards for his sixth goal of the season.
With both teams using wing-backs there were forays galore down the flanks.
Leeds goalkeeper Paddy Kenny was twice relieved to see shots by winger Chris Burke zip just wide of the far post and Lee Novak had a goal disallowed for offside.
The energetic McCormack felt Dan Burn had fouled him inside the box but his claim for a penalty was waved aside by referee Paul Tierney.
McCormack was involved again when Leeds doubled their advantage in the 33rd minute.
Austin picked McCormack out on the left with a superb pass from inside his own half before sprinting into the goalmouth to send a downward header past Randolph from the Scot's deep cross.
Then, on the stroke of half-time, Smith released McCormack whose shot was parried by Rodolph back to Smith and the former Oldham striker reacted quickly to fire home Leeds' third.
Birmingham manager Lee Clark replaced defender Burn and striker Novak with Andrew Shinnie and Nikola Zigic at the start of the second half, but Leeds continued to create openings and Austin shot straight at Randolph when he should have scored.
With Shinnie improving Birmingham's midfield play, the visitors were unfortunate not to cut the deficit three times in a five-minute spell.
Tom Lees made a goal-line clearance to deny Burke, Kenny brilliantly tipped over a sizzling drive by Zigic and David Murphy smacked a free-kick against the near post.
Leeds survived again when Sam Byram cleared Burke's angled shot off the line but with 74 minutes gone Austin and youngster Alex Mowatt combined on the left for Smith to head his second goal and Leeds' fourth.
Austin received a deserved ovation on being replaced by Michael Brown just before the final whistle.
Birmingham were left glancing over their shoulder at the relegation places while much-improved Leeds completed their biggest home win since a 4-0 victory over Shrewsbury Town in the Capital One Cup first round in August 2012.