He initially did missionary work for two years in the parish of the Sacred Heart in Boston, which served a congregation of Italian immigrants mostly from Genoa.

On July 19, 1899, he was assigned as assistant pastor of Our Lady of Pompeii Church, established in 1892 by Fr Pietro Bandini in New York City's Greenwich Village on Bleecker and Carmine Streets.

[2] His spiritual care and leadership were put to test on March 25, 1911, when he had to respond to the tragic Triangle Shirtwaist Company fire, which claimed the lives of 146 female employees.

Father Demo organized a campaign to buy a nearby property and with the help of a leading Italian American architect, Matthew Del Gaudio, build a new church and rectory.

He died in 1936 in Greenwich Village in New York City, and thousands of parishioners and friends, including Mayor Fiorello La Guardia, paid their respects.