Police responded to the shooting on Hanford Street at 10:17 p.m.
Three suspects had confronted the 62-year-old woman outside of her home and then forced her back inside at gunpoint, police said.
At some point a fight broke out between the suspects and one of the residents, and one of the suspects opened fire.
Only the woman was struck by gunfire, but her son had been hit in the head and also required medical attention. The three suspects ran away, police said.
The woman and her son were taken to a hospital. The woman required surgery and the extent of her injuries is not yet clear. The son's injuries are not considered life-threatening.
Anyone with information about the shooting has been asked to call Union City police at (510) 471-1365.