It was just after 12:30 p.m. when the call came in for smoke coming from a two-unit apartment on Union Street.
When firefighters arrived, they found smoke coming from a storage unit located beneath the upper deck.
They were able to open the unit and put out the fire.
Authorities are still not sure what caused the incident.
"It's currently under investigation," said Brewer Fire Lt. Erik Tourtillotte. "The Fire Marshal's Office has been called and the downstairs unit may be displaced for a time."
Crews from Eddington, Holden, Orrington, Dedham and Bangor assisted Brewer at the scene.
Fire officials say the building suffered minimal damage, thanks to the area's quick response.