One of the injured is a police officer who was responding to the fire before midnight Tuesday when something exploded and he suffered a minor hand cut, says Mount Vernon city spokeswoman Shannon Haigh.
Three others from the Cascade West apartments were treated for smoke inhalation at Skagit Valley Hospital.
Witnesses say the fire engulfed the building in minutes.
"I went out on the balcony, it was huge, already spreading. It was ridiculous," said Kate Wood, who escaped the fire and warned other tenants.
Kirk Deogracia and his family -- including his week-old daughter -- barely made it out.
"To step out of your apartment and the hallway to be full of smoke -- all I thought of was getting everyone out of there and out of that," he said.
Haigh says a man was found about two hours later and was flown to Harborview Medical Center in Seattle with burns to his airway and other parts of the body.
The three-alarm fire caused heavy damage to the top two floors of the three-story building.
"I know it's just stuff, but I work two jobs, it's really hard," Wood said.
About 75 of the displaced residents are being sheltered at nearby Skagit Valley College with help from the Red Cross.