Beginning the night on 99 UCL goals, Lewandowski was presented with an ideal chance to bring up his century after just nine minutes, when he chested down Pedri’s delightful clipped ball into the box, before being clattered to the ground by onrushing Brest goalkeeper Marco Bizot.
Bosnian referee Irfan Peljto pointed to the penalty spot and Lewandowski sent Bizot the wrong way for goal number 100 - joining Cristiano Ronaldo and Lionel Messi as the only players to reach that century.
Bizot was more successful the next time he came off his line, smothering a shot from Fermín López, who saw another chance drift wide midway through the half.
A deflected Raphinha effort flew narrowly wide before López again was denied by Bizot, who kept out a powerful header with his legs, as Les Pirates barely got out of their own half in the first 45 minutes.
Having thrown away a two-goal lead late on at the weekend, Barça were intent on wrapping up all three points quickly here, but López was still losing his personal battle with Bizot - thwarted with another strike after a lovely backheel from Lewandowski to set him up.
Then, on the hour mark, the goalkeeper flapped a Jules Koundé cross straight to Dani Olmo, but his clean strike was cleared off the line by Brendan Chardonnet.
However, five minutes later, Olmo gained revenge, collecting a Gerard Martín pass, shimmying past Chardonnet and slotting coolly past Bizot for Barcelona’s second.
With 15 minutes remaining, Éric Roy’s side thought they had reduced the deficit with their first shot on target, only for Mathias Pereira Lage’s fine strike from a tight angle to be ruled out for offside.
That was the closest Brest came to becoming the first French side in 25 years to go unbeaten in their first five UCL matches, while it looked as though Pablo Torre’s late miss would see Hansi Flick’s side narrowly miss out on becoming just the fifth side to win four successive UCL matches by three goals, only for Lewandowski to add his second of the night with a composed finish in stoppage time.