Though his father had hoped that Jacob would have a literary career, Jacob wanted to be a carpenter. When he was 16, he became fond of Elisabeth Gjørtz, the 12-year-old adopted daughter of the owner of the company for which he worked as an apprentice carpenter. The father disapproved of the boy’s blundering attentions, and Riis was forced to travel to Copenhagen to complete his carpentry apprenticeship. Riis returned to Ribe in 1868 at age 19. Discouraged by poor job availability in the region and Gjørtz’s disfavor of his marriage proposal, Riis decided to emigrate to the United States.

Riis immigrated to America in 1870, when he was 21 years old, seeking employment as a carpenter. He first traveled in a small boat from Copenhagen to Glasgow, where on May 18 he boarded the steamer Iowa, traveling in steerage. He carried $40 donated by friends (he had paid $50 for the passage himself); a gold locket with a strand of Elisabeth’s hair, presented by her mother; and letters of introduction to the Danish Consul, Mr. Goodall (later president of the American Bank Note Company), a friend of the family since his rescue from a shipwreck at Ribe.

After five days, during which he used almost all his money, Riis found work as a carpenter at Brady’s Bend Iron Works on the Allegheny River above Pittsburgh. After a few days of that, he began mining for increased pay but quickly resumed carpentry. Learning on July 19, 1870, that France had declared war on Germany, he expected that Denmark would join France to avenge the Prussian seizure of Schleswig, and determined to fight for France. He returned to New York, and, having pawned most of his possessions and without money, attempted to enlist at the French consulate, but was told that there was no plan to send a volunteer army from America. Pawning his revolver, he walked out of New York until he collapsed from exhaustion; on waking, he walked to Fordham College where a Catholic priest served him breakfast.

Riis had for some time been wondering how to show the squalor of which he wrote more vividly than his words could express. He tried sketching, but was incompetent at this. Camera lenses of the 1880s were slow as was the emulsion of photographic plates; photography thus did not seem to be of any use for reporting about conditions of life in dark interiors. In early 1887, however, Riis was startled to read that “a way had been discovered to take pictures by flashlight. The darkest corner might be photographed that way.” The German innovation, by Adolf Miethe and Johannes Gaedicke, flash powder was a mixture of magnesium with potassium chlorate and some antimony sulfide for added stability; the powder was used in a pistol-like device that fired cartridges. This was the introduction of flash photography.

Recognizing the potential of the flash, Riis informed a friend, Dr. John Nagle, chief of the Bureau of Vital Statistics in the City Health Department who was also a keen amateur photographer. Nagle found two more photographer friends, Henry Piffard and Richard Hoe Lawrence, and the four of them began to photograph the slums. Their first report was published in the New York newspaper The Sun on February 12, 1888; it was an unsigned article by Riis which described its author as “an energetic gentleman, who combines in his person, though not in practice, the two dignities of deacon in a Long Island church and a police reporter in New York”. The “pictures of Gotham’s crime and misery by night and day” are described as “a foundation for a lecture called ‘The Other Half: How It Lives and Dies in New York.’ to give at church and Sunday school exhibitions, and the like.” The article was illustrated by twelve line drawings based on the photographs.

Riis’s first team soon tired of the late hours, and Riis had to find other help. Both his assistants were lazy and one was dishonest, selling plates for which Riis had paid. Riis sued him in court successfully. Nagle suggested that Riis should become self-sufficient, so in January 1888 Riis paid $25 for a 4×5 box camera, plate holders, a tripod and equipment for developing and printing. He took the equipment to the potter’s field cemetery on Hart Island to practice, making two exposures. The result was seriously overexposed but successful.

Riis had already been thinking of writing a book and began writing it during nights. (Days were for reporting for the New York Sun, evenings for public speaking.) How the Other Half Lives, subtitled “Studies Among the Tenements of New York”, was published in 1890. The book reused the eighteen line drawings that had appeared in the Scribner’s article and also seventeen reproductions using the halftone method, and thus “[representing] the first extensive use of halftone photographic reproductions in a book”. (The magazine Sun and Shade had done the same for a year or so beginning 1888.)

Riis tried hard to have the slums around Five Points demolished and replaced with a park. His writings resulted in the Drexel Committee investigation of unsafe tenements; this resulted in the Small Park Act of 1887. Riis was not invited to the eventual opening of the park on June 15, 1897, but went all the same, together with Lincoln Steffens. In the last speech, the street cleaning commissioner credited Riis for the park and led the public in giving him three cheers of “Hooray, Jacob Riis!” Other parks also were created, and Riis was popularly credited with them as well.

Riis wrote his autobiography, The Making of an American, in 1901. His daughter, Clara C. Riis, married Dr. William Clarence Fiske. His son, John Riis (1882–1946), served in Gifford Pinchot’s new United States Forest Service from 1907 to 1913 as a ranger and forest supervisor on national forests in Utah, California and Oregon. He chronicled his time in the Forest Service in his 1937 book, Ranger Trails. Another son, Edward V. Riis, was appointed US Director of Public Information in Copenhagen toward the end of World War I; he spoke against antisemitism. A third son, Roger Williams Riis (1894–1953), was also a reporter and activist. In 1905, Jacob Riis’s wife Elisabeth became ill and died. Riis remarried in 1907, and with his new wife, Mary Phillips, relocated to a farm in Barre, Massachusetts. Riis died at the farm on May 26, 1914. His second wife lived until 1967, continuing work on the farm, working on Wall Street and teaching classes at Columbia University. Riis’s grave is marked by an unmarked granite boulder in Riverside Cemetery, in Barre, Massachusetts.