Top scorer McCormack hit the first goal and had a major part in two others as Leeds romped to their biggest home win since a 4-0 eclipse of Shrewsbury in the Capital Cup at the start of last season.
Birmingham's defence was a shambles and it was ruthlessly exposed as giant striker Matt Smith helped himself to two goals and all-action skipper Rodolph Austin was also rewarded with a downward header past overworked Birmingham keeper Darren Randolph.
Delighted Leeds manager Brian McDermott said: "The board backed me on Ross when they were offered good money for him and didn't take it.
"His goal was special, he worked very hard and his partnership with Matt Smith was very good."
It was only Leeds' second win in seven games and McDermott added: "We managed to get some width into the team and that was important.
"My teams have always had width, as anyone who saw Reading when I was there will tell you.
"We spoke beforehand about having a winning mentality. The more games you play, the more you learn how to win and we got closer to where we want to be."
McDermott explained that he took Austin off just before the end so the Leeds crowd could show their appreciation of his efforts.
"He led by example and I wanted him to see what the fans thought of him," said the Leeds boss.