Backed by Hollywood superstars Ryan Reynolds and Rob McElhenney, Wrexham have enjoyed a dream ascent from the National League to the Championship thanks to three consecutive promotions.
After another summer of heavy investment in the transfer market, the Welsh side looked set to make a flying start to their ambition to reach the Premier League.
The visitors led 1-0 at half-time thanks to Josh Windass' penalty and could have been further in front as Windass also hit the bar.
Southampton, relegated from the Premier League last season, were under the orders of Will Still for the first time and the former Lens coach had a telling hand to play in the turnaround.
Left-back Ryan Manning was introduced off the bench in the final quarter and the Ireland international secured a memorable three points for the Saints with a goal and assist.
Manning's brilliant free-kick on 90 minutes levelled before he crossed for Jack Stephens to fire in the winner six minutes into added time.
Charlton also scored a stoppage time winner to beat Watford 1-0 on their return to the Championship.
Coventry and Hull played out a 0-0 draw.
Birmingham, bankrolled by an ownership group containing seven-time Super Bowl winning NFL quarterback Tom Brady, began their campaign with a 1-1 draw at home to Ipswich after conceding a controversial stoppage time penalty.