De Rossi was born into a happy country family in Voltaggio, near Genoa. When he was 10 a wealthy Genoese couple, who had become enchanted by the boy on their holidays, persuaded his parents to let them take him home and have him properly educated.
In Genoa he so impressed some Capuchin monks that they arranged for him to go to Rome to study, where he lived with a cousin from his birth family who was a priest.
In spite of childhood ill health, he was ordained in 1721 and worked for 40 years among the sick and among homeless women and girls. His cousin left him a large house when he died which he gave over as a refuge for homeless women and prostitutes. He also worked as a prison chaplain and was famous as a confessor. He died following a stroke in 1763 and was canonized in 1881 by Pope Leo XIII. His saint's day is 23 May.