Nigeria forward Lookman did not feature in Atalanta's first three league matches after agitating for a transfer to Paris Saint-Germain.
But the 26-year-old netted the decisive strike in stoppage time at the end of the first half, jinking through the Fiorentina defence before shooting past David de Gea, seconds after Charles De Ketelaere headed Atalanta level.
Lookman, the hat-trick hero from last season's Europa League final triumph, also floated in a pinpoint cross for Mateo Retegui to head in Atalanta's first equaliser in the 21st minute.
"Yeah, I'm happy to be back, in front of our crowd, in front of our fans to be playing football," said Lookman to DAZN.
His immediate impact is great news for coach Gian Piero Gasperini, whose side host Arsenal on Thursday night and entertained their home fans in the newly-completed Gewiss Stadium.
"We're very pleased to be playing in the Champions league this season," added Lookman.
"We deserve it so we have to demonstrate that again on Thursday with the same drive, the same mentality that we showed today."
Atalanta, whose most recent appearance in the Champions League was in 2021, played their first three Serie A fixtures away from Bergamo to allow the club to put the finishing touches to their redeveloped home.
They are eighth on six points, three ahead of 14th-placed Fiorentina who twice went ahead in the first half through Lucas Martinez Quarta and Moise Kean but are yet to win under new coach Raffaele Palladino.
Earlier Genoa snatched a 1-1 home draw with Roma in the final seconds to continue their positive start to the season.
Koni De Winter headed home Genoa's leveller in the sixth minute of added time at a packed Stadio Luigi Ferraris to deny Roma a first win of the campaign.
Artem Dovbyk looked to have shot Roma to victory when he prodded home in the 37th minute after Niccolo Pisilli's effort was spilled by Pierlugi Gollini.
But De Winter's first Serie A goal gave Genoa their second late home draw of the new season after also stunning champions Inter Milan in their opening fixture.
Genoa are 10th on five points, level with AC Milan and two points ahead of Roma who sit in 15th.
Later Inter have the chance to reclaim the league lead ahead of Juventus and Torino, who drew 0-0 with Lecce, when they make the short trip to Monza ahead of their Champions League opener at Manchester City on Wednesday.
Napoli and Romelu Lukaku are at Cagliari where the Belgium striker suffered racist abuse in 2019.
