Archie Joe Darby was attacked by the Staffordshire Bull Terrier-type dog as mum-of-two Jade Rogers carried him into a relative's home in Colchester, Essex.
Her other son Daniel-Jay, who was 22 months old at the time of the incident, suffered life-changing injuries but would also have been killed had Mrs Rogers not managed to wrestle him to safety.
The owner of the dog was the boys' aunt PC Clare Ferdinand, a serving police officer for Essex Police.
Essex coroner Eleanor McGann praised her for the mum-of-two's "bravery" in fighting off the aggressive animal.
Det Insp Garry Biddle from Essex Police added Ms Rogers did a "heroic job" in removing the dog.
He told the inquest: "The dog attacked Daniel. The dog went straight for his head and started biting and shaking his head.
"The dog dropped Daniel and grabbed Archie out of her arms. The dog then shook Archie around the room and then dropped Archie - at that stage she knew instinctively Archie was dead.
"She managed get him (the dog) into the conservatory and barricade him in."
An ambulance was called and CPR was attempted but Archie was pronounced dead at the scene at 4.02pm.
A post-mortem examination revealed the baby died of severe head injuries.
Police have previously stated the incident is 'unlikely' to bring criminal charges as the dog was not a banned breed and the incident took place in a domestic home.