Crews are now trying to ventilate the building at 400 Ortega Ave., spokesman Lynn Brown said. The blaze was reported just before 9:30 a.m. and was controlled around 11:30 a.m.
Twenty-two people who were evacuated from the building, part of a three-building complex, won't be able to return home until firefighters determine the building is safe.
The fire left at least three units uninhabitable and the Red Cross has been dispatched to work with those displaced, Brown said.
Investigators are waiting for clearance to enter the building, and have not yet determined the cause of the fire. However, Brown said a contractor was doing work in the unit where the fire appears to have started.
The flames traveled up to the third floor through the "pipe chase," the area inside the walls that contains the pipes, he said.
Additional units were called to assist Mountain View firefighters as the blaze jumped from two alarms to three, Brown said.
"Apparently the fire extended into the walls between apartment units, which made it very difficult" he said of firefighting efforts.
More than 40 firefighters from nearly a dozen agencies including Palo Alto, Moffett Field and the Santa Clara County Fire Department, helped battle the blaze, Brown said.