Lon Carson and his wife, Kimberly Carson, rushed to the cat's aid and both were bitten by the dog at about 10:15 p.m.
"I would have broke the dog's neck if I could," Lon Carson said.
Police officers arrived and put the dog in the back of a cruiser.
The attack was witnessed by the Carsons' two daughters, ages 4 and 8, with whom the cat slept, he said. "They didn't need to see this. They were both screaming," Lon Carson said. "It was part of the family."
Community Rescue Service medics arrived to treat the injuries to Kimberly Carson and Lon Carson and then offered what aid they could to the badly injured cat.
Arrangements were made for emergency veterinary services at Park Circle Animal Hospital and Hagerstown Fire Police volunteer Todd Sunderland took the cat there with Lon Carson holding the pet in his lap.
The ambulance took Kimberly Carson to Washington County Hospital for treatment of her badly bitten hands. The cat, Frosty, died on Lon Carson's lap as Sunderland pulled into the animal hospital's parking lot, Lon Carson said.
Frosty was raised by the Carsons after being abandoned last summer by its mother in an alley near their home, Lon Carson said. They had bottle-fed the kitten until it was old enough to be weaned, he said.
When Lon Carson returned home from the animal hospital, he saw the dog being walked home by its owner.
Lon Carson said an SPCA official did not tell him whether the dog had its rabies shots.
SPCA officials could not be reached for comment.