Second of six sons and second of eight children born to Dominico Castagnetto and Angela Maria Raggio.

The 1870 census lists John living with his family in Mariposa, he was a quartz miner, except his father lived in a separate household while a Bartholomew, one year younger than John's mother, was the head of the household..
John married the sister of George Dulcich, but he declared himself single in the 1910 census. Since John killed his wife's brother in self defense in 1904 there might have been a divorce.

Census and ancestry trees give his birth as 1852, his stone states 1849. 1870 census lists his birth place as Italy.

Mariposa Gazette, December 14, 1912: DIES SUDDENLY
Word reached Mariposa Thursday morning of the sudden death of John Castagnetto, a prominent farmer and gardener of Hunter's Valley. Deceased was a native of Massachusetts and about 63 years of age at the time of his death.
Mr. Castagnetto had been a resident of Hunter's Valley continuously for the past several years, but a number of years of his life were spent in Coulterville and vicinity, where he followed the occupation of mining. Deceased was an honest and upright man who had many friends who will regret to hear of his death. He also leaves to mourn him several sisters and a brother, namely: Mrs. Cademartori of Hornitos, Mrs. David Lord of Merced, and Mrs. A. Dulcich and Daniel Castagnetto of Hunter's Valley.
Death was due to a hemorrhage of the lungs and occurred about 6:30 a.m. Thursday. The interment will be in the Bear Valley cemetery today, where he will be laid to rest near the remains of a departed father, mother and brothers.

CASTAGNETTO, John - December 21, 1912 Mariposa Gazette:
The death of John Castagnetto, which occurred suddenly last Thursday morning, was a shock to the entire community. Heart failure is supposed to have caused his death. He was about 61 years of age. The deceased possessed many good qualities, the most admirable that of rendering his assistance to the needy and sick and always ready to sympathize with the afflicted. Mr. Castagnetto has left to mourn him one brother, three sisters, and numerous nieces and nephews, besides a large circle of friends, who sympathize deeply with the bereaved family. The funeral took place at Bear Valley and was largely attended.

Stanislaus County Weekly News, 29 July 1903 excerpts
John was acquitted for the slaying of his brother-in-law, George Dulcich at his home in Hunter's Valley July 21st.
John had the reputation of a respectable, peaceful and generally law abiding man, while the man who was killed was just the antithesis, being quarrelsome and unruly when drunk.
The killing was the outcome of an old grudge. Dulcich frequently accused Castagnetto of mistreating his wife, Dulcich's sister.
On July 31st Dulcich went to Castagnetto's house and asked his sister where he could find John, telling her that he was going to kill him. The man was drunk at the time. Mrs Castagnetto told him that her husband was in the barn and the would be slayer went out to look for him, shooting off his revolver at some chickens on the way. Unable to find Castagnetto he returned to the house and saw Castagnetto leaning from a window with a shot gun in his hands. Dulcich pointed his revolver at his brother-in-law and snapped it but it did not fire and before he could again pull the trigger Castagnetto emptied the contents of the shot gun into Dulcich's body, the latter falling immediately and dying a short time after.
The Coroner's inquest and jury returned a verdict of justifiable homicide.

Cause of Death: Verdict of Jury: death caused by hemorrhage of the lungs