Educated by his father, a teacher in the Latin School, he spent a four-year apprenticeship with a carpenter in Copenhagen after attending Ribe Katedralskole.
Riis learned about journalism when still a boy by helping his father prepare a weekly paper. He came to New York in 1870 and picked up various jobs as a laborer for three years. In 1873 he landed a job as a police reporter for the New York Tribune, and worked for the Evening Sun about 1888. He also did work for Associated Press.
Riis' photographs aided his work as a social reformer in putting through a statute providing for playgrounds. He was a leader in the tenement reform movement and helped establish Mulberry Bend Park and the Jacob A. Riis Neighborhood House. With the support of Theodore Roosevelt, then of the New York Police Commission, he was also instrumental in abolishing the police lodging-house system.
Riis never aimed for the aesthetic but nonetheless created an unequaled record of New York's slum life from about 1885 to 1902, including both its misery and its vitality. His dedication prompted Theodore Roosevelt in 1903 to call him "the ideal American." In his work as a police reporter on New York's Lower East Side, Riis employed the newly invented flashbulb technique to illuminate the dark hallways and drab squalor of tenement conditions, using gelatin dry-plate negatives and silver prints.