Paul Parry, Kevin McNaughton and Joe Ledley all found the net and City can now prepare for Wembley.
Parry made his return in attack instead of Steve Thompson - the only change from the side victorious in that Cup semi-final.
Tykes keeper Luke Steele had to block a Jimmy Floyd Hasselbaink shot with his legs in the fourth minute as the Bluebirds went on to the attack in the early stages.
It was Hasselbaink who went close ten minutes later after dispossessing Ayarza Diego Leon, but his sizzling drive swerved outside the far post with Steele beaten.
Peter Enckelman went full-length to push a Jacob Butterfield shot round the post on the half-hour, but it was the Bluebirds that were still setting the pace and three minutes later Peter Whittingham rattled the crossbar with a well-struck angled drive.
City went ahead a minute before the interval with Parry's 11th goal of the season. Glenn Loovens ran through the visitors defence before switching the ball to Ledley. He in turn passed inside to Parry and his drilled shot went low into the corner of the net.
City doubled their lead four minutes into the second period. McNaughton chipped the ball into the danger area from the byline, Steele was unable to gather and the ball dropped over the line with Ledley rushing in to make sure.
Ledley took advantage of a slip by Dennis Souza to score the third in the 63rd minute, side-footing beyond Steele and into the corner of the net after Souza had missed his clearance.
It was one-way traffic as the Bluebirds swarmed down on Barnsley's goal and they gave game time to all three substitutes as they finished the season in style.