McCormack's first-half goal delivered a blow to the home side's survival hopes.
Fresh from their 2-1 victory over fellow strugglers Charlton, Barnsley were looking for another three-point haul to boost hopes of pulling off a late-season rescue act.
But they never seriously threatened to drag themselves back into this Yorkshire derby after falling behind to McCormack's early goal.
It was a game played at a fast pace from the off, with Leeds having the better of the opening exchanges.
Barnsley keeper Luke Steele was called into action to keep out a header from Matt Smith and Leeds' Luke Murphy was the next to threaten when he had a header blocked.
McCormack gave the visitors a 16th-minute lead after Smith's knock-down found him free inside the area and he fired low past Steele for his 29th goal of the season.
Leeds defended well during the opening 45 minutes, keeping the hosts largely at bay.
Barnsley's solitary effort on goal before the break was a looping header from Dale Jennings which was easily gathered by keeper Jack Butland.
Tomasz Cywka also volleyed wide after connecting with a cross from Stephen Dawson.
Early in the second half, Jennings' tame shot was easily saved by Butland.
Barnsley's Kelvin Etuhu then saw his shot blocked before putting the follow-up well wide.
McCormack tried a 20-yard effort which went over and his team-mate Noel Hunt, on as a substitute in place of Marius Zaliukas, had a shot blocked.
As the visitors continued to press forward in search of a second goal, Smith put a header on target following Tom Lees' free-kick but it was straight at Steele.
Barnsley continued to be frustrated in their attempts to force an equaliser, with Jennings seeing his header saved by Butland.
Barnsley manager Danny Wilson, who had earlier sent on Paddy McCourt in place of Kelvin Etuhu, withdrew midfielder Dawson and replaced him with striker Nick Proschwitz.
After Liam Lawrence put a 25-yard effort over, Hunt threatened to find a way through at the other end following a defensive lapse before finding his route to goal blocked by Jean-Yves M'voto.
In the five minutes of stoppage time added by referee David Webb, it was Leeds who looked the more likely scorers and McCormack fired in a low drive which went wide of Steele's left-hand post.