Firefighters responded to The Gibbs Villas just before 6 p.m. Thursday, where four units in the condominium were on fire.
Tuolumne County Sheriff's Office said the apartment building, located at 19849 Villa Road, was evacuated.
Don Smith, president of the homeowners association, said everyone got out safely and no one suffered serious injuries.
The Gibbs Villas is a gated community at the top of Gibbs Road consisting of 14 condominiums and eight single family homes.
"It's a cul de sac. There's only one way in and one way out," Smith said.
Battalion Chief Kent Cunningham of Tuolumne County Fire Department and Cal Fire, said the fire was reported at 5:50 p.m. as a multi-family dwelling fire. The first three engines on scene were city of Sonora and five to six firefighters with his unit.
He said the firefighters knew there were people inside and they went door to door to clear the building before they started fighting the fire. The first units on scene were Engine 751 of the City of Sonora and Engines 761 and 511 from Tuolumne County Fire Department with Cal Fire personnel on board.
There were no civilian injuries and no one suffered smoke inhalation, he said. One firefighter suffered possible dehydration and heat related injuries and was transported to the hosptial.
"We lost four units to heavy fire damage," he said.
The fire burned through the roof in two units. The cause is undetermined, Cunningham said. At about 8 p.m., he said investigators had not yet been able to get inside the building.
American Red Cross was on the scene talking with Assistant Fire Chief Andrew Murphy. As firefighters battled the blaze, residents gathered down the hill from the complex.
The point of origin was inside one of the units. Firefighters had the fire under control by about 7:30 p.m. and kept it confined to the structure.
"There were 10 displaced so far," Cunningham said. "There be more."