City employees throughout the Civic Center complex in Fairfield watched Bryan Cavanah, 21, of Fairfield, as he drowned the dog in Civic Center pond at 1000 Kentucky St. at about 10:15 a.m.
He brought the dog into the pond and dunked it into the water and held its head down repeatedly until it stopped coming up, police said.
The dog, a brown female pit bull with a chain around his neck, belonged to Cavanah's father.
When police arrived the dog had already died and Cavanah admitted to killing the dog and said he didn't know why he did it, according to police.
Cavanah was arrested for felony cruelty to animals and violation of probation. He was on probation in Solano County for assault with a deadly weapon, police said.
He was booked into Solano County Jail and is being held without bail.
Officers used a boat from the public works department to recover the remains of the dog.