Arriving back on home soil needing to make up a 2-1 deficit from the first leg, Metz made a rod for their own back inside the opening five minutes as Pape Diallo saw red, but the 10-man Grenats surprisingly managed to wipe out Saint-Etienne's aggregate lead through Lamine Camara and Georges Mikautadze.
However, Leo Petrot's response for Les Verts forced an additional 30 minutes, where substitute striker Ibrahima Wadji swept home the equaliser on the evening and the goal that ended Saint-Etienne's two-year exile from the top flight, condemning Laszlo Boloni's men to a heartbreaking demotion despite an enormously courageous effort.
Needing to go gung-ho in their bid to right their first leg wrongs, the last thing on Boloni's bingo card would have been losing an attacker inside the opening five minutes, but that is exactly what transpired for Les Grenats.
Left winger Diallo caught Dennis Appiah with an atrociously high tackle, and after the referee checked the monitor and saw the Metz man's studs connect with the right-back's thigh, he had no option but to brandish the red card.
A Saint-Etienne onslaught was seemingly imminent, but Metz instead managed to rise up in the face of adversity and went ahead in the 17th minute, losing his marker and tapping in from Matthieu Udol's cutback into the six-yard box.
Valiant Metz effort in vain as Saint-Etienne return to big time
Camara immediately riled up the bouncing Stade Saint-Symphorien crowd, and it had the desired effect, as Mikautadze won a penalty off of fellow attacker Irvin Cardona and made no mistake from the spot for his 14th goal of the season.
As admirable as Metz's efforts were, they were only ahead on aggregate for all of 10 minutes, as a Saint-Etienne corner was headed onto the back post, where Petrot caught his marker napping and finished clinically with a side-footed volley.
The visitors' pressure was unrelenting henceforth, but the 10-man Grenats held out until half time and survived a huge scare in the 66th minute, when Saint-Etienne had a goal controversially disallowed.
Substitute Yvann Macon capitalised on a collision between Ibrahim Sissoko and Metz goalkeeper Alexandre Oukidja to roll the ball into an empty net, but upon a review, the referee deemed the Saint-Etienne striker had fouled Oukidja.
Olivier Dall'Oglio's side failed to make their numerical advantage count before the full-time whistle blew, and a couple of chances passed both teams by in the first half of extra time, where extreme fatigue was prevalent.
However, just when it appeared that penalties were inevitable, a slick 117th-minute link-up between Appiah and Nathanael Mbuku saw the former slip in the latter in the right-hand side of the box, and the Augsburg loanee cut back for Wadji to nonchalantly stroke home the promotion-clinching goal.
As Saint-Etienne join Auxerre and Angers back in the top flight for 2024-25, Metz will be playing second-tier football alongside Lorient and Clermont next term.