Letty McQuay is suffering from bite wounds to her face, head and leg. She was transported to Children's Hospital in Oakland by helicopter after the attack.
Witnesses said someone from the house rushed the girl outside as soon as paramedics arrived.
"I saw the girl was running with the baby in her hands and kind of crying. And just everybody just trying to get the baby," neighbor Alex Miranda said.
Animal control officers seized the pit bull from the home soon after the attack. They said the bites to the little girl are severe.
It's unclear what led up to the mauling, but officers say several family members were home when it happened.
According to neighbors, John McQuay and Maritza Jiminez-McQuay are Letty's parents.
Lt. Joe DeCosta of Contra Costa County Animal Control said: "There's always the potential we'll be investigating whether or not they had knowledge of the dog having prior aggressive history."
If that was the case, there's potential they could face some legal criminal charges, but DeCosta said that at this point it doesn't appear that's the case.
At least one neighbor was surprised by the attack. Hosay Jamili owns three dogs. She says she has never seen the pit bull act aggressively in any way.
Jamili said the pit bull was very calm and subdued. "They always had great control on him and he's never lunged at our dogs," she said.