The Reds' unbeaten start to the Sky Bet League One season comes to an end at Coventry.
A second half Jordan Willis header condemned Barnsley to a first league defeat of the season this afternoon.
Following a soft free-kick, Willis was on hand to head home what proved to be the winner.
Daniel Stendel had given a first league start of the season to Jacob Brown, whilst both Adam Davies and Kenny Dougall returned to the starting lineup after their time away with Wales and Australia respectively.
It was another fast Reds start, with Kieffer Moore seeing a close-range header fizz just over the bar from Mamadou Thiam's left-wing cross.
With just seven minutes on the clock, Barnsley survived a huge scare at the back as Conor Chaplin was slipped in one-one-one with Davies, but the Sky Blues striker somehow missed, slotting the ball a yard wide.
With 18 minutes gone Stendel's side were gifted a glorious opportunity of their own as a defensive mix-up saw Brown racing in on goal, he tried to bend the shot around the 'keeper and low, but Lee Burge got something on the shot which then hit the outside of the post and out.
Brown had a second chance soon after when after a free-flowing Reds move Thiam slid the ball across goal, Brown tried placing his shot first-time on the run, 10 yards out but his effort went a foot wide.
The game became scrappy and fractious for the remainder of the first half, with Dujon Sterling nearly turning a Reds cross beyond his own 'keeper only for Burge to grab hold of at the last second.
HALF TIME: Coventry 0-0 Reds
Thiam went close early in the second half but his bending effort from the edge of the box was superbly tipped over the bar by Burge.
Sadly, the Reds were under a lot of pressure from there on until the winning goal arrived.
Chaplin had the ball in the net which was ruled out for offside, but Mark Robins' side were attacking the Barnsley backline at pace and they struggled to cope.
George Moncur was introduced on 65 minutes, replacing Mike Bähre but the scrappy, stop/start nature of the game continued.
After an eventful 72 minutes, Brown was withdrawn, Victor Adeboyejo taking his place in the Reds frontline.
Despite the changes and a few half-chances, it was the home side who grabbed the crucial goal.
Referee Kettle deemed a perfectly legal tackle by Dimitri Cavaré as a foul and awarded the Sky Blues a free-kick on the left touchline, 40 yards from goal.
The set-piece was whipped in, the Reds failed to clear the initial delivery, it was headed back across goal and in came the unchecked Willis to nod home what would be the winning goal that ended the Reds' unbeaten run, and extended the 95-year wait for a win in Coventry.
FULL TIME: Coventry 1-0 Reds
Davies, Cavaré, Pinnock, Lindsay (McGeehan 82), Pinillos, Thiam, Bähre (Moncur 65), Dougall, Mowatt, Brown (Adeboyejo 72), Moore.
Attendance: 11,465 (1,829 Reds)