In 1877 Riis became a police reporter for the New York Tribune.
Riis was determined to use his journalistic skills to write about poverty. He constantly argued that the "poor were the victims rather than the makers of their fate."
In 1888 Riis worked as a photo-journalist for the New York Evening Sun.
Among first photographers to use flash powder, which enabled him to photograph interiors and exteriors of the slums at night.
He also became associated with what later became known as muckraking journalism.
Over the next 25 years, Riis wrote and lectured on the problems of the poor.
Riis also wrote over a dozen books, including Children of the Poor (1892), Out of Mulberry Street (1898), The Battle With the Slum (1902) and Children of the Tenement (1903).