The match-winning strike from Lewandowski made him the fastest player to reach nine goals in the Spanish top flight in the 21st century, with this just his seventh game since arriving from Bayern Munich.
Real Madrid can reassert themselves as leaders when they face Osasuna on Sunday, but the sight of Barca at the summit, however fleetingly, points to the prospect of a fierce title battle.
There was plenty to satisfy coach Xavi, including a first domestic league start of the season for Ansu Fati, even if his team had to settle for a slender victory margin.
Lewandowski put Barcelona ahead in the 20th minute, collecting a pass from Ansu Fati and racing in from the left. He cut back cleverly onto his right foot to make space, cracking a powerful low shot into the far corner.
The Pole had another chance shortly after but could not apply power to his finish after being picked out by Sergio Busquets with a smart first-time pass.
Mallorca went close to a leveller when Jaume Costa tested Marc-Andre ter Stegen after good persistence on the right flank from Antonio Sanchez.
Sanchez forced another save from Ter Stegen soon after the break with a vicious cross-shot, while at the other end Franck Kessie sent a 25-yard drive just wide after good work from Fati.
Kessie and Fati made way midway through the half as Xavi brought on Pedri and Raphinha, emphasising Barcelona’s squad depth.
Pedri, Gavi and Raphinha almost combined for a fantasy goal, but Barcelona ultimately did not need a second.