The Bluebirds came from behind twice against the Championship's bottom side to continue Harris's unbeaten start in the role.
It looked as if Barnsley were on course to move off the foot of the table until a late Cardiff fightback, which was completed thanks to Tomlin's 95th-minute winner.
"When you've got a magic man like Lee Tomlin, you stay in those games, you've got players who, in any moment, can win it," said Harris.
"He's got the quality to open up a defence with a pass, like for Danny's goal, or he's got the quality to hit a ball dropping from the sky on the full into the bottom corner.
"It's not just any moment either you're talking about the 95th minute of a game, when he's ran 12km again, which, last week, was the most he's ever run in his career. He's getting those rewards now.
"First and foremost let me be very honest, that wasn't a great performance from us.
"I thought we were a little bit lethargic and I'll hold my hands up for it. We've had a tough week, when we don't have a midweek game I have to find a way of working the group and getting them up to speed because maybe I pushed them too hard."
Chaplin played a key role again two minutes after half-time when he fired the ball across the goal towards Bamba Diaby. The defender outmuscled Lee Peltier and saw the ball go in off his side for a 2-1 lead.
But Danny Ward came off the bench and latched onto Tomlin's through ball to put Cardiff back on level terms.
And Barnsley were hit with a sucker punch in the 95th minute when Tomlin volleyed in from the edge of the box to snatch victory.
Tykes manager Gerhard Struber believes his biggest task is changing the mindset of his confidence-drained players.
"The feeling now is like being hit in the head," he said. "I am very frustrated and disappointed. The mindset was not to go on and win the game after they equalised.
"We had control of the game in the first 20 minutes of the second-half, we had the lead, but after the equaliser I saw the mentality of the previous month in my players.
"The mindset was not to go on and win the game, the personality is not to go and get the next goal, many players were thinking it would be difficult.
"This change is my job and responsibility, I need to change the mindset so the players believe they can win, but now it's only a mentality situation.
"My job now is to help the boys for the next game on Wednesday, we are going in the right direction, we are very close to winning games, it is only a matter of time."